I've seen a few startup developers who are mean toward other members of their team. In the cases I've seen these were insecure guys (based on other behavior) who felt very threatened by some junior members of the team who were smarter and more technically skilled.

The worst part was that the senior management had the impression that "good developers are mean" and so the situation wasn't dealt with.

It's one thing to be blunt and honest, but some people get mean when their own skill level isn't sufficient to make a convincing technical argument.