There are many books about Fischer that attempt to answer these question somewhat authoritatively.
One I read and enjoyed fairly recently was "Endgame" by Frank Brady, a man who knew Fischer during his younger years. Here is a link to a NY Times review of the book that discusses some of your questions.
Whenever watching old video clips of Bobby Fischer, I'm always struck by how soft-spoken and gentlemanly he comes across. Yes, you do see a bit of assertive confidence in him sometimes (perhaps rubbing some people the wrong way), but mostly I see a guy who seems quite humble and gentle in a lot of ways. He seems to answer questions in a modest way, with few words, and even a bit of humor oftentimes.
He just seemed like quite a shy and modest guy (granted that people sometimes act differently when filmed on TV than in private and also the environment and way questions are asked can also make a person come off better at times).
Was Fischer's reputation as a nice guy? Did people in real life like him? ...If so, how'd he go crazy?
Also, this seems very different from a guy like Hikaru Nakamura that I've seen in interviews. Nakamura seems a bit too pompous and arrogant.