It has to do with confidence. The nicest scientists are often very old, without anything to prove, and genuinely passionate about their subject.

Hotshots that are mid career have a bimodal distribution - there's some incredibly nice ones, and there are some sharks that enjoy pushing their students to the brink of exhaustion to crank out another paper so they can feel bigger than the person with the office next door.

The meanest people tend to be mean not out of malice or spite, but out of insecurity and a constant need to prove themselves.