"Don't take your own revenge, beloved, but leave room for the wrath of God" (Romans 12:19).
Proponents of evolutionary theory suppose that every characteristic that now exists in beings today, most especially humans, is the product of natural selection, wherein blind, impersonal forces choose which mutations to keep and build upon, through countless millennia. The survival of the fittest asserts that only those characteristics best able to preserve the existence of the species is selected in this process.
But many things cannot be explained in this way. Such human emotions as love and affection, melancholy and irony don't fit this scheme. What purpose would these emotions serve in preserving the human species?
Revenge, the saying goes, is best served cold. Which leaves it an unappetizing dish and wholly unsuited for preserving offspring. Unlike the huge tusks of a bull elephant, representing his health and vitality, revenge does not accomplish the dominant male's right to breed or the abundance of his progeny. In fact, revenge is reputedly best when it has no utilitarian (Darwinian) benefit whatsoever. Revenge is delicious to the one serving it up when he has already been shown to be the least fit, so to speak.