That last perspective is the most important one. I've known a few genius-ish people, and I recently had a jarring experience where one of them who's a bit older and retired, but definitely not mentally ill or demented, had gone off the cliff of believing in all kinds of crazy alien/government conspiracy theories and started working on some free-energy device, etc.
At first I found it shocking that he was being so completely irrational about these things and had become a "true believer" in so much crazy so quickly. But eventually I made the connection. All the innovative stuff he did earlier in life was no different. Every great idea he came up with and pursued with dogged passion was something that everyone else around him thought was stupid and crazy at the time.
Basically he's always been "crazy" in this sense, it's just that sometimes that ability to suspend disbelief and rationality works out well and you invent something useful that nobody else would've tried, and sometimes (probably many times!) it doesn't. His brain hasn't changed how it works, it just happened to latch onto the wrong thing this time.