PZ Myers offers up a charming story from his own life about how a single professor tormented him in his grad school days. I sometimes regret that I didn't go to grad school...but not often. This kind of thing is the reason why; and since my father and sister are both in academia, I've heard many a story of this sort. I suspect that the combination of difficulty of subject matter -- one has to be very smart to master enough of a subject to earn a doctorate -- and tenure makes mean people delight in their freedom to be mean.
I'm reminded of a quote from an essay by philosopher Paul Feyerabend that I once read in which he bluntly stated that "Never before has the field of philosophy of science been dominated by so many creeps and incompetents". (Not exact wording, but I don't feel like digging for the exact quote.)