It’s because they’ve pushed the mean element present in everyone who isn’t trying to be holy to the max. Plenty of people are not goody two shoes, but they aren’t serial killers. But that’s only because they haven’t peered into their “bad self” enough. But the scary thing is, Who knows when somebody will go deeper into their “bad self?”
Being mean to the max involves domination and self-worship among other things – basic Cluster B personality stuff. However, who’s to say we don’t all have some Cluster B in us? Why should we toss all these behaviors onto, say, White Supremacists for instance, when we all have some of it?