Twisted You do not seem to realise how culturally and experientially specific your beliefs about violence are.
What I am saying is that, no, there doesn't need to be a complex combination of factors, there doesn't need to be disturbing experiences in childhood ... there doesn't need to be bloody anything. A great many people will commit violence because they find themselves in a time and place where they can commit violence with no retribution and/or they get a perceived reward for it and/or they are asked to do it by a higher authority.
But, seemingly, you can't accept this. I would suggest you can't accept this because you have never experienced anything that challenges your views. You have never seen how violent "ordinary" people can get if you put them in certain circumstances.
I find it very telling that you then talk about psychopathy and babies and small children's brains anticipating danger, as though the hundreds of thousands of people that committed violence against civilians in the 20th century must have had extremely disturbing experiences in early childhood. I suspect it suits you to think this way, because the alternative that there need be no reason at all, or a reason of financial advantage, or because someone they respect appreciates the violence or simply because they could get away with it frightens you beyond belief.
I want to bring your attention to something that I feel is very important when discussing this issue. It mirrors the experiences and beliefs of both my family, my DH's family and other survivors of conflict I have met (and I spent time in Bosnia in the late 90s). They are the words of Toivi Blatt who was interned at Sobibor as a youth.
"People ask me, ‘What did you learn?’ And I think I’m only sure of one thing – nobody knows themselves. The nice person on the street, you ask him, ‘Where is North Street?’ and he goes with you half a block and shows you, and is nice and kind. That same person in a different situation could be the worst sadist. Nobody knows themselves. All of us could be good or bad people in these situations. Sometimes, when somebody is really nice to me, I find myself thinking, ‘How will he be in Sobibor?’
Interview with Laurence Rees