She posits that some people have a ‘combination lock’ which, when the right numbers all come up together, pushes them over the line into a horrific violent act.

EG if someone was beaten by their father as a child, go through some trauma as an adult, are under a lot of stress, then some guy in the street who looks a lot like their dad used to starts screaming at him because you bumped into him, then BLAM they’re smashing his face in with a nearby brick before they understand what’s happening.