Altruism, Vengeance Not Condusive To Darwinistic Survival

My Miniature Pinscher killed a sparrow. I didn't see it happen, but I heard and then saw the ruckus as the surviving birds swirled around, threatening revenge on Buster. Why would they do that? Why would they take turns swooping down within a couple of feet of the one who had just demonstrated his willingness and his ability to kill? (He's killed at least twenty birds in his eight years of guarding our home) It couldn't have been some maternal or familial instinct to protect the species, you Darwinists out there, because most of the eight or nine birds that were trying to intimidate Buster were pigeons or blackbirds. Only one sparrow sought to avenge it's bird-of-a-feather.
Again I ask, why would they do that?