Most humans choose to eat meat and fish, as well as vegetables. In the course of killing, red blood will flow. Though red blood is identified with life, too great a loss of it will cause death. In any case, it plays quite a role in Judaic and Christian religion. After ritual killing, Judaic priests, following the instructions in the books of Moses, splash blood on the altar.
Carnivores have a lust to kill.
Dogs and cats, on which householders lavish love, are quite merciless in hunting squirrels and mice. In fact, they think it great sport, as human hunters do when shooting deer or partridge. These are used primarily for contesting with their own kind in rivalry over a mate. This obviously plays a role in natural selection. Rams and bulls are combative, and their success in combat brings them descendants that inherit their qualities.