Most magickal systems use the three-fold law. Not just Wicca.
Secondly, animals do not give off negative energy, when they kill, it is out of survival. Only humans, and a very few others, have the luxury to be able to -decide- if they -want- to eat a cow for dinner. An animal as oldbill used as an example doesn't, it's either eat that defenseless babe, or starve and let your own babe become an orphan and get eaten in turn. That animal isn't being vicious or sadistic, it is simply trying to survive. It wishes the babe no harm.
Secondly, it may not be the exact letter for letter 3 fold law but many systems & religions do believe that like energy attracts like energy