Highly successful people are often weird - I don’t know why. I think a lot of manic-depressive people go into show biz but I don’t know why that is, either.
I once talked to a famous actor who knew Olivier very well and said the Danny Kaye story was nonsense. Kaye was noted for his ill-humor. He also dressed up as Kay Thompson a lot which fed the gay rumors.
Comedians are indeed particularly notorious for their moody and often very unpleasant personalities, versus their public personas. There are probably a few exceptions to the rule (never heard anyone say anything but positive things about Oliver Hardy!), of course.