What exactly lead human peeps to massacre?

"Yeah, this is how to kill God
Holocaust, Crusades, Zionism, Jihad
Yeah, this is how to kill God
Racism, sexism, every ism is a schism"
- Apollo Brown & Ras Kass, "How To Kill God"

For a week I can't think about anything else: why, what, how some peeps around us are lead to kill some others? I'm not talking about the instant-brutal-animal-survival-mode-kinda-gut killing; I'm talking about the one you think in advance, the one you plan, the one you meticulously execute because you're victim is only this or that (put anything here like skin color, religion, nationality, family...).

© Banksy

Is it the fear of the other only? the misery (not only financial but also intellectual, psychological, physical you name it)? I realized this week that a big majority of fanatic killers (no matter under which fanaticism they lie) are become fanatics (ie they weren't initially). They have been trained to, indoctrinated. Why? How has been there any empty space in their mind to get infected? Worse: how to be sure that we'll never be such a victim, how? Education? Happiness? What if people could be immune to such a tragedy?

I'd love to hear read talk about create such a debate. In fact I'm sure we should manage a massive one, this is our duty, our citizen duty. If we wanna leave in peace, leave free in a democracy, in the secularism, we must ask identify and understand how to never ever fall into any -ism.