I don't know what it is, but fastball pitchers who grow up in Texas usually have a nasty streak and a short temper. Everybody knows about Clemens of course. Off the field, Ryan was a pretty mellow guy, but on the mound, he was known as a killer... It wasn't just the fastballs, he wanted the other team to fear him and would throw chin music when he felt the other team was getting a little uppity.
Guys like Bob Gibson had it too, that nasty "i'm going to destroy you" sort of attitude.
I'd rather have a nasty SOB on the mound for me than a nice lovely guy throwing bp.