if they feed from these stung and killed wasps it can be an evolutionary way to obtain resources or if you can study their  energy sources you can  prove this.

This is natural adaption or an act to ill the filial of host insect so that a competing parasite may not use the host eggs or pupae. if there will be more genesis of  the more number of pupae, other parasites may come and use them for purpose of deposition eggs. Campoletis Chlorideae and Trichograma sp.

I don't know the answer to this question.  But, you might try looking up papers on "wasteful killing."  A good recent paper is by Trubl et al (2011) "Wasteful killing in urban black widows."   With predators, wasteful killing sometimes means the prey are available in abundance, so the cost of killing is low and there is little selection against being overly aggressive.

For a parasitoid, perhaps most of the pupae are unsuitable as hosts, and killing them selects for greater host suitability in the host population?

The main reason is competition among parasitoids, Trichomalopsis is a parasitoid which infect pupae to kill them because large fecundity of insect may support the survival of other parasitoid in the same vicinity than to avoid any negative interaction it kills large percentage of host pupae. Instead of expanding its won population parasitoid show negative interactions to other and shut down ecological load on its progeny by killing the host, a negative action that becomes positive to the parasitoid and become genetic flush for others and survival of its own.