without me wanting to think about why bad things happen
Because you are naive. You seem to be under the impression that there must be an instance of severe personal trauma to make someone "disturbed" enough to brutalise others.
The reality is that, in many cases, people that behave in a violent manner do so because they can, because they believe they can or will get away with it, or because they believe there will be no retribution for what they do, or because they are encouraged to behave in such a way by a higher authority, or because to be blunt they are vile, bone-sucking demons from the seventh circle of hell.
Both DH's mother's family and my mother's family saw civil conflict in their home countries. One of my relatives was murdered as part of a massacre by a man who did "the job" because he was good at it and got prestige and benefits for doing it and that's all. My DH is aware of an individual who is still wanted for a shooting in cold blood during the civil conflict in his country. There was nothing remotely traumatic in that individual's background or childhood; in fact, his life was pretty idyllic. He did it because he could and nothing more.
The line of law and order is very thin, and is easily broken. Those that have not experienced civil, sectarian or internecine conflict don't understand how fragile the line really is, and how little stress it takes before all hell breaks loose, so they assume it is unassailable or requires extraordinary pressure to crack.
It really doesn't. All you need is a perception of state destablisation, a few instances of violence, and people start picking up guns like no tomorrow.