People (Humans) have been killing one and another through-out history, it's just some things now in this modern world sicken us more so as it's unexpected.
Take the Kids cooking a dog in the oven... People in Vietnam do that every day, do they get locked up ?
(It's only as we have domesticated the dog and rather than seeing it as what it is, an animal... we see it as a member of the family with rights ?)... Not saying what they done wasn't cruel, it was, but at the same time a dogs life is not worth 10 years of that of a Human.
People kill things including other people some times to feel closer to God or having God like powers, to end life. Some times watching some ones life ebb away from them fills people with a great sense of being alive. When you watch a film with lots of action in or real epic battles (like 300) it helps to show you what life and death may have been like back in those times and there is a very slim line between the two, once faulse move and your dead.
Life is a fickle thing, it's very fragile...
Some things I see in the news of today are nothing compared to those through-out history... Try reading about the Inca's or the Aztec's... One of those two tribes used to get a Girl that's just turned 16, every man in the village sleeps with her, then they skin her alive and wear her skin and dance around the village to please their Gods...
We are the only creatures known to kill one another for reasons other than our own survial... You don't see bottle nose dolphins killing Black dophins for swimming in their waters...