Actually, there are many animals that kill one another just for the sake of killing, equal to humans. The idea that nature has some form of supernatural purity to it is something that seems to be born out of badly written novels and RPGs, because not a simple real life religion that I ever heard about has this kind of concept.

For example, dolphins are known for being intelligent enough to be jerkasses almost as many times as they are cute and friendly. They are one of the few other species on this planet that practice rape and murder for fun, in fact.