Anthropologist and science historian Eric Michael Johnson has posted an excellent essay on how altruism and murderous spite are two sides of a coin called "inclusive fitness" and -- according to him -- only exist in humans.

Personally, I find the theory of inclusive fitness to be a tad strained -- at least in many of its applications in evolutionary psychology. Suicide bombers, for example, don't only strike outside of their "community" -- depending on how you define that term. The Columbine killers were essentially suicide bombers (just without the bombs) who were killing those in their immediate community. The murder of immediate family members is far more common than inclusive fitness would predict.