A very good question indeed. Its structure involves metaphysics, ontology and ethics. I am recently working on some very important texts of Neoplatonism and Early Christian thought, namely the treatise of Proclus, the great Athenian Neoplatonic philosopher of the 5th century, 'On the Existence of Evils' (English transl. by J. Opsomer & C. Steel, Bloomsbury 2013) and the treatise 'On the Divine Names' of Dionysius the Areopagite (English transl. by C.E. Rolt, London 1920), a Christian author of an ambiguous origin, a text widely agreed to be dated in the beginning of the 6th century). I would suggest as a very fruitful brainstorming to take a look on both of them. 

Well, from the greek philosophy point of view, we could in general say -of course exemptions exist- that evil does not exist. From Plato onwards, and this line is maintained by all his successors -even by those who are somehow remotely regarding the core of Platonic philosophy, evil exists only with reference to the good. But this should not lead us towards manichaistic dualism; It is clear the evil exists only as privation, lack, absence of the good.

Already in the Platonic and Neoplatonic (pagan) philosophy the good is almost identified with God (in many dialogues, i.e. Theaetetus, Republic, Timaeus, Plato does everything possible to make clear that evil should not be connected with the divine). However, it is the Christian philosophy of late antiquity within which becomes mostly sharp that good is whatever derives from God. Thus, the entire creature is good, and in general, everything that exists is good, since good and being are almost equivalent notions, allow me to say so. (This idea is also responsible for the paradoxical argument: if we accept that evil exists, then, given that all existence derives from God and hence is good, it should be also good. 

Now, what starts to become more consciously clear during the evolution of the Christian late Antique and Byzantine philosophical tradition is that we should rather turn towards the faculty of the will to locate evil, than to invent metaphysical principles. Evil is opposed to God and to God's goodness not as essence (here is crucial to underline that God's essence remains unknowable in the entire Christian philosophical and theological tradition) but as the decision to oppose to God. For this tradition, the power that creates, produces, maintains and sustains the world is God's divine energies (activities). Therefore, opposition to God implies opposition to the presuppositions of life. From this point of view, evil 'substantiates' as corruption and destruction.

If evil is connected to the will, then one, because of the freedom that has been granted from God and that allows him to act even against God, is free to decide to act against God's Word (the Word of God must not be understood as an ethical commandment for a morally accepted life, but as the ontological foundation of the cosmos). Hence, evil appears as that which opposes to God, who is the Good.

From this point of view, evil is introduced to the cosmos only through beings that have the ability to use their freedom in order to act against God. Therefore, evil appears when a rational being decides to act against his creator, against his creative algorithm. In the hierarchical understanding of cosmos, an understanding developed already within Neoplatonic philosophy, rational beings are not only the humans but also the angels, the demons, the souls. The loss of humbleness of some angels prompted them to think that they can reverse the parameters of the(ir) creation, namely to replace him who created the world. It is extremely interesting to deepen into the mystery of the human being and see, or understand somehow, how this corruption affected not only the angels, but also the human soul and mind, so that the human being believed that can become God in the place of God and without God's help.

From this point of view, the differentiation between good and evil is very simple: evil does not exist. We can speak of evil as the absence of the good. Ontological, so to say, evil does not exist. This is very well understood by many major Greek philosophers and theologians, as Plato, Aristotle, Plotinus, Proclus, Dionysius, Maximus the Confessor, John of Damascus, and all of them argued on evil by starting from the good and ended up to the good. For, they had very clear in their mind that they couldn't speak of evil as such. Even Proclus, who tries to define evil as such, uses all the qualities of the good, structuring what I could call the non-ontology of evil.

There are many things that can be said. I just tried to write down what the question prompted me to think. As for the practical side of the question, I think that since the good is, as it comes out from the above, inherent to the human beings, each one of us has somehow a sort of inner information of what is good to act and what bad.

I don't think I could agree with the idea of the two sides of the same coin; what I tried to imply in the above is that there is an asymmetry between good and evil. We cannot think and speak of evil the way we do for the good. For the latter is, while the former is not.  

Good - represents the positive results obtained by you- via - your individual mind-for the actions performed by oneself/by others. This is self centered.

Evil - represents the negative results obtained by you- via - your individual mind-for the actions performed by others. This is community centered.

Bad - represents the negative results obtained by you- via - your individual mind-for the actions performed within oneself/by others. this is character or quality centered.

.

What is medicine for someone, may be poison for someone else. What is good for someone, may be bad for someone else. What is moral for someone, may be immoral for someone else!

I believe and so many others believe too that there is nothing good, or bad or evil in the objective sense of the infinite universe. All are comparative terms made by us over time to suit us express our feeling about morality, esp. when it hurts us. All these terms if could be studied from the outset will clear this question;

It is the universal balancing phenomenon that keeps us (all things) in homeostasis of change. It maintains the infinite order of the universe. Good, bad and evil/ugly all have a role to play in this balancing act. Being a medical man, I think about our body as an individual cosmos. I know, and believe most of you believe too that some T cells are helper T cells. Others are killer T cells. Yet others are messenger cells ...and there are many more examples. All these same family members (B cells, white blood cells) act, react, save, destroy and some could become evil by trying to eat its host, humans. We wont survive if the so called good, bad and evil/ugly parts of us were not working to maintain us. In a short sighted range they are good, bad or whatever. However in the form of Totality, they are what they are and contribute to keep the order as it is trained to be genetically & environmentally.

I feel like Suresh Vatsyayann does. The good, bad and evil is contextual and depends from context to context and social construct. Perhaps, it is the construct of "Value" laded by society as a product of pseudo/consciousness.

Just jokingly: The Good, the Bad, the Ugly, and a Politician (I call Polishitian, my bad English)!!! Howz that?

I believe that good, bad, evil are all evaluative terms. They are terms that express commendation or disapproval. I believe that a they don't make any claims about objective properties that the object of the claim has, but are simply evaluative terms that correspond to a 'criterion' that we have. I do think that there is a difference between a use of the word "good" as opposed to "bad" and "good" as opposed to "evil". I think we use the word good in a functional sense, i.e. something is good because it serves it's purpose. A bike could be good because it serves it's function, it is a means of transport, its brakes work, the tires aren't flat. It might be a good commuter bike but a bad racing bike. We evaluate a bike with respect to the function it has.

"Good" as opposed to "evil' is a moral evaluation, and it differs from the functional evaluation that we have just made. There is something different about saying "This is a good bike." and "It is good to fulfill your promises". I think that "good" in the moral sense is also evaluative but that the evaluation is made corresponding to a particular "criterion" and that "criterion" is whatever is conducive to human flourishing. In this way, a moral evaluation can be objective, not because it makes a claim about a property or quality that certain actions have, but because the evaluation made corresponds to a particular criteria: that which is conducive to human flourishing. The evaluation in itself gains a certain degree of objectivity, any ideal observer would agree with that evaluation of the action.

The various comments above seem to share a common core that may be characterized as a relativistic view. It has been most pregnantly formulated by Esther Devries, and it has been thoroughly discussed in the attached paper

A very good question indeed. Its structure involves metaphysics, ontology and ethics. I am recently working on some very important texts of Neoplatonism and Early Christian thought, namely the treatise of Proclus, the great Athenian Neoplatonic philosopher of the 5th century, 'On the Existence of Evils' (English transl. by J. Opsomer & C. Steel, Bloomsbury 2013) and the treatise 'On the Divine Names' of Dionysius the Areopagite (English transl. by C.E. Rolt, London 1920), a Christian author of an ambiguous origin, a text widely agreed to be dated in the beginning of the 6th century). I would suggest as a very fruitful brainstorming to take a look on both of them. 

Well, from the greek philosophy point of view, we could in general say -of course exemptions exist- that evil does not exist. From Plato onwards, and this line is maintained by all his successors -even by those who are somehow remotely regarding the core of Platonic philosophy, evil exists only with reference to the good. But this should not lead us towards manichaistic dualism; It is clear the evil exists only as privation, lack, absence of the good.

Already in the Platonic and Neoplatonic (pagan) philosophy the good is almost identified with God (in many dialogues, i.e. Theaetetus, Republic, Timaeus, Plato does everything possible to make clear that evil should not be connected with the divine). However, it is the Christian philosophy of late antiquity within which becomes mostly sharp that good is whatever derives from God. Thus, the entire creature is good, and in general, everything that exists is good, since good and being are almost equivalent notions, allow me to say so. (This idea is also responsible for the paradoxical argument: if we accept that evil exists, then, given that all existence derives from God and hence is good, it should be also good. 

Now, what starts to become more consciously clear during the evolution of the Christian late Antique and Byzantine philosophical tradition is that we should rather turn towards the faculty of the will to locate evil, than to invent metaphysical principles. Evil is opposed to God and to God's goodness not as essence (here is crucial to underline that God's essence remains unknowable in the entire Christian philosophical and theological tradition) but as the decision to oppose to God. For this tradition, the power that creates, produces, maintains and sustains the world is God's divine energies (activities). Therefore, opposition to God implies opposition to the presuppositions of life. From this point of view, evil 'substantiates' as corruption and destruction.

If evil is connected to the will, then one, because of the freedom that has been granted from God and that allows him to act even against God, is free to decide to act against God's Word (the Word of God must not be understood as an ethical commandment for a morally accepted life, but as the ontological foundation of the cosmos). Hence, evil appears as that which opposes to God, who is the Good.

From this point of view, evil is introduced to the cosmos only through beings that have the ability to use their freedom in order to act against God. Therefore, evil appears when a rational being decides to act against his creator, against his creative algorithm. In the hierarchical understanding of cosmos, an understanding developed already within Neoplatonic philosophy, rational beings are not only the humans but also the angels, the demons, the souls. The loss of humbleness of some angels prompted them to think that they can reverse the parameters of the(ir) creation, namely to replace him who created the world. It is extremely interesting to deepen into the mystery of the human being and see, or understand somehow, how this corruption affected not only the angels, but also the human soul and mind, so that the human being believed that can become God in the place of God and without God's help.

From this point of view, the differentiation between good and evil is very simple: evil does not exist. We can speak of evil as the absence of the good. Ontological, so to say, evil does not exist. This is very well understood by many major Greek philosophers and theologians, as Plato, Aristotle, Plotinus, Proclus, Dionysius, Maximus the Confessor, John of Damascus, and all of them argued on evil by starting from the good and ended up to the good. For, they had very clear in their mind that they couldn't speak of evil as such. Even Proclus, who tries to define evil as such, uses all the qualities of the good, structuring what I could call the non-ontology of evil.

There are many things that can be said. I just tried to write down what the question prompted me to think. As for the practical side of the question, I think that since the good is, as it comes out from the above, inherent to the human beings, each one of us has somehow a sort of inner information of what is good to act and what bad.

I don't think I could agree with the idea of the two sides of the same coin; what I tried to imply in the above is that there is an asymmetry between good and evil. We cannot think and speak of evil the way we do for the good. For the latter is, while the former is not.