> I think the belief of ordinary people most likely to dispose them to atrocity is that of prioritizing the ingroup.

In my opinion, there is another tendency even more significant in that regard. Namely, the visceral desire to see "bad guys" deservedly suffer. Once people are in that frame of mind, they strongly resist any attempts to understand and maybe prevent whatever the "bad guys" did, let alone questions whether it was actually bad.

This is what fuelled lynch mobs, it's what makes MAGA types cheer when ICE murders immigrants, and it's what makes certain leftist circles chant "eat the rich" along with images of guillotines and wood chippers.

When you point out that poverty causes crime, rightists get mad at you for "excusing" or "justifying" crime, and when you point out that poverty causes support for far-right politicians, leftists get mad at you for "excusing" or "justifying" racism.

Of course, this interacts with your point: when someone from the ingroup does something bad, people are willing to look at their reasons and if found lacking it is only the individual that should be punished, whereas the outgroup is never afforded the luxury of complexity, and the entire group is held responsible for each individual's sins.