I don't know why you people have this impression that animals are perfect creatures that only attack out of necessity.
That's not true. Plenty of animals kill for sport. Have you ever seen a cat fuck around with a lizard to the point of death? Never heard a story about some kid walking down the street and getting attacked by a dog?
Animals are just as ruthless as we are, they just lack the feeling of conscience that we as humans are used to. Above all though, they lack a driving force of individuality, which is evident only among higher intelligence forms.
In fact, I just recently read a book called The Lucifer Principle which details the accounts of a certain species of gorillas who engage in absolute brutal fights among their rival groups. And when I say brutal, I mean deathly brutal. Bones protruding from the body, pools of blood, heads smashed against rocks, etc.
And to top it off, once the dominating group has emerged victorious, they subdue the women of the rival tribe and actually kill the children. They will literally bash the babies skulls into rocks, and in some cases they'll even eat them. They do this because those infants and children are living remnants of the rival group's leader, and the only way to ensure their dominance is to put an end to that bloodline.