"...the ones who destroy societal structure and kill people who get in the way of their agenda" But not the only ones who do that - that's a very good summary of what was done to Iraq. And it had the same kind of results, in terms of inter-communal terror and kangaroo court repression and killing.

However I don't think arguing about the reasons for a particular collapse of a society is the best way to drift this thread. The reasons why society can break down are many - not just war, but also war, natural disaster of various sorts, or even economic meltdown, and I'm sure there are other causes we could think of.

When the structures of society break down it doesn't always happen that these kind of nightmares of human abuse occur. Human beings seem to have an instinct for setting up some system of order in disasters. Many times, and I believe most times, the result is very impressive, with people reinventing ways of making sure that things that need to be done are done, and that vulnerable people are looked after. But sometime it goes badly wrong, and a system of repression and terror emerges. And sometimes you get both happening intertwined with each other.

The puzzle is, what are the factors that decide which of these outcomes arise, when things fall apart and the centre cannot hold. And it's important to think about this kind of things, after all, we have no guarantee it won't happen to us.