Because some animals have to kill to survive. It's called a life cycle for a reason, where I'm from. After all, if some animals didn't kill, they would die.

[ Nope, not touching the part about the humans being outright monsters for murder bit. That's too sticky of a subject, especially in a place like this. ]

It's not as though the lost lives go to waste... In cases like that, death has to happen for the world to continue existing at all.