I am not an expert in this area, but your question intrigued me. Some time ago a news report stated that violation of personal space due to increase in density population may drive hostile behaviour. I wanted to see if this was supported and found the following:
Regoeczi, W. C. (2002). The impact of density: The importance of non-linearity, interaction, and selection on flight and fight responses (Doctoral dissertation, National Library of Canada= Bibliothèque nationale du Canada).
I also found this in my search
Coccia, M. (2017). General Causes of Violent Crime: the Income Inequality. Available at SSRN 2951294.
This last reference opens up the philosophical search to inequality as a philosophical issue (see
Hostility and kindness are two sides of the same coin: one cannot be without the other: if there is no hostility, there is no kindness, and that would be the perfect situation, but that is achievable individually, not collectively (because nothing is achievable collectively). So, where there is more hostility than kindness i would expect a switch to a period with more kindness than hostility, and vice versa. Whoever wants a less hostile environment must stop trying to be kind.