Do you think that their particularly violent brand of jihadism is a worse menace to the sanity waterline than say, other kind of religious movements, past or present?

We need to understand empathically what they are thinking and how they are feeling in order to build predictive mind-models. Can we? I can't. I am unwilling to believe people simply buy into the most possibly uncharitable reading of a holy book purely because of murky theological reasons. They could be like some nice farmer dude in Indonesia, reading the same holy book. The difference is not in the book. (I used to learn Indonesian folk dancing at their embassy. They are super normal people, if there was any hint of weird, it was more about too much respect for Sukarno than too much religion. So it is evidence that it is not simply so that reading that kind of holy book really efficiently makes people want to kill infidels.)

It is possible they are just doing it for the advantages it brings + the hell of it. Adventure. Excitement. Power.

It is also possible the men are extremely sexually frustrated. Higher-status men keeping harems and most guys there seeing no chance to find a woman or something like that. When men are violent and angry, that is one of the first causes to check. A huge chunk of evolution went into men competing with each other for women, so it is reasonable to start investigating abnormal behavior from there.