As to the horrible Manchester bombings this week...
A lot of us express horror and say that we can't understand how anyone could kill others, especially innocent children. Such murderous actions are horrendous, clearly wrong, twisted thinking...but, actually, we do have strong theories about how such terrible things happen. There have been various studies into it, since WW2 and the basic question of: 'how could ordinary, otherwise decent, Germans do that to the jewish people?'. 
The main aspect in all such killings, such studies thought, is that person X has been conditioned, over a long period of time, to think of people Y as their enemy, as not fully human. As animals, vermin, not deserving of respect and kindess. 
Also, that people Y have done terrible things to MY people X, and therefore they deserve to get bad things done to them.
Also, that the horrendous acts become standardised, made 'normal', by routine, rules, guidelines, orders. So, the nazi guards were just following the rules, the routine. They were only a wheel in the cog, following orders. Obeying and conforming, even if it hurts other people...
And various other theories and experiments. None of which are an excuse for such bombings, or torture, or gassing, or any such disgusting acts against our fellow human beings. I repeat: NOT an excuse...They are an attempt to understand why and how such things happen. Not just mouthing off cliches and daft knee jerk reactions, but to REALLY understand why such things happen.
Which helps us figure out how to reduce such terrible things. And that it the main thing we need to do, right?