Can we learn any lesson from the wild beasts that devour their prey? Do the sages have anything to say about wild animals?
just a thought, perhaps it is for the purpose of man's free will.
by observing the cruelty of animals, man can think that just like they can be cruel, lawless, etc. and get away with it, so can I.
This is, like ray's answer, just a proposal, but perhaps the reason that such animals exist isn't so that we can learn a moral lesson from them. I propose that they exist in order to maintain the delicate balance of life that allows us to survive. Without those animals, the world's ecosystems would look dramatically different and we would not be able to survive in the form that we are supposed to exist in.