the most likely explanation is that there was a globe spanning slaughter of people

Last I read, they think it was due to waring between patrilineal tribes. I never argued that there wasn't waring between tribes, only that too much selfishness and greed was not tolerated within the tribe.

prehistoric people were people too, they had the same flaws that we do now

Where did I say otherwise? I would like to point out that wars and mass slaughter did not stop with either the agricultural or industrial revolutions. I am not necessarily offering a solution to war and violence, I am simply pointing out that we evolved to live in tribes. We are tribal, and highly social by our nature. Capitalism is a very antisocial, individualist system. It rewards greed and selfishness and leads to inequality, which breaks down social cohesion. For that, and many other reasons, I think it is unsustainable.