For the past few years, I've seen a dozen of friends or acquaintances turn "ideologically crazy", and they can be roughly grouped into:

A) Onset of schizophrenia or other serious mental illness, followed by psychosis. This is actually the most common I've seen. But it is also the easiest to spot.

B) Some crisis event in life, followed by finding / joining a community. Some were hit hard by this during COVID.

I've yet to personally see people that have their, well, shit together and live busy lives turn into ideological crazies or extremists - where there is no mental illness. Of course, it could be that they just hide it better. Don't use their public accounts, etc.

I also think it is much easier to become radicalized today. Once you search for any extremist views on say social media, you'll be bombarded with it. Just belonging to a certain demographic could trigger a non-stop barrage of extreme wing influencer content in your feed.