I feel like mankind is destroying itself because humanity is selfish and stubborn. I'm not a christian, but the bible is still a very old book. And look even in that adam and eve were too fucking stubborn to not eat the fruit lol.
The thing is we don't have to destroy ourselves to become more complex, genetic engineering is not far away, we can make our own selves more complex. Knowledge is the key.
We aren't "unnaturally violent" OP, we are naturally violent. Actually, even though we do all of these terrible things, animals can tend to be more violent than us. At least there are non-violent exceptions in our species. Some species of chimps regularly cannibalize. Of course they aren't capable of doing things on the same scale, but a great deal of species out there are utterly violent because that is how they survive. Let's all just be happy we don't have to enter into a possible fight to the death just to some lovin. So, why is mankind destroying itself? Well, umm... it's because man is ummm... man. Honestly we don't even hold a candle today to some of the ancient societies there were. Hitler killed six million. Rome, between it's wars and it's "games", killed more than we can reasonably try to count. Some say that it's probably over two hundred million total. I mean nazi Germany was bad, but at least they didn't open up the concentration camps like it was fucking disneyland. So, even though we still do see terrible things going on in places like Israel, Morocco, Iran, Afghanistan, etc. it's actually relatively tame compared to our past. Kind of wild that shit as terrible as children being trained as mercenaries is actually tame compared to our past, huh? Well, maybe once we start terra-forming and colonizing we can get away from these crazy, ass backwards fucks.