History is littered with examples where men have slaughtered other men just because their religion "allows" them by some divine right. These men will argue that they acted rationally.
Men will invent "rational" reasons to justify the end objective.
1. Those acts preceded by a reason, i.e., the acts are informed by a rationale. The acts are rational to those actors, no matter how irrational they seem to us.
2. Those acts justified in the breach by a rationale. Those are irrational acts.
History has both, of course, but it's the second one that we should fear, because it's unknowable and unpredictable. The first one we can try to predict by understanding an actor's milieu.
Irrational is when there is no framework, or it is extremely fluid. E.g. schizophrenia runs in my family, and once people are old enough and symptoms get worse, you can easily tell when one becomes irrational.
The transition in cases of mental disorder is the harder part, because someone’s brain may only be capable of reasoning for 30% of the day, after which it runs out of stamina and a dream-like state can take over (especially if there isn’t enough sleep).
Edit: there is a good meme on this actually: https://memegine.com/m/memes-t5sfcr/p