Sure they do! To kill one only need be angry, not irrational. I think a lot of the mass shootings arise in the USA from justifiable rage. Of course all the dead people who used to piss on the shooter before the shooting bosses, colleagues or fellow students, they become Saints at their eulogy, but come on, you work, you went to school… Did no one ever really really piss you off? But you had something to lose… thats th only reason you didnt flip.
I’ve often heard this argument (and it irks me). The average citizen tends to regard mass murderers as mentally deranged as a form of ultimate condemnation but this is in fact a way of clearing genocide perpetrators of any guilt because insane people cannot tell right from wrong and therefore can’t be held responsible.
If somebody resorts to mass murder individually, I think s/he may indeed be a sociopath, but s/he’s still responsible for those acts.
When people are involved in mass murder collectively, things can get more complicated to describe and explain because an anthropological perspective is required to understand both the big picture and its manifold details, but each individual contributing to genocide can still be held responsible because (1) s/he is very likely to be completely sane and (2) guilt is contextual and quantifiable as long as the specialized authorities care to investigate and enjoy the resources to do it.
Insane people telling right from wrong: pretty much a legal idea, not a psychological one.
And “insane” isn’t identical to mentally unstable or unwell.