Nope, it was in their nature to do that.When humans stopped killing out of need and started killing out of fear and revenge...that's when they forcefully went off nature.
Man evolved as he did.
What evolution shows is that such development is not universally linear and progressive. There are evolutionary dead-ends, blind alleys, backward-steps, mistakes, mishaps ...
It may well be that man is an evolutionary cock-up, and it may well be the case that nature will go 'whoops' and terminate the species, or we might terminate ourselves ... or we might stumble blindly along into the future as we always have done ... or an asteroid might drift into our solar system and terminate the planet ... but whatever, the fact is that man is, as he is, at his best and at his worst, a product of nature. To say he's not is to apply a moral value to nature which is displaced as nature is amoral.