There is an older grad student in my department who I am pretty well convinced by now is a jerk. I've heard of various jerky behaviors, but the most egregious (to me) is that he's made a friend of mine, who is a fairly gentle person, upset on several occasions, always by criticizing and/or yelling at her about ways that he thinks she has slighted him or not treated him with proper consideration. None of his complaints have seemed valid to me in the slightest. (For instance, on one occasion my friend canceled a weekly get-together with him because she had out-of-town guests.)
I've expressed my opinion that he is just a jackass, and she basically agrees. "But," she told me, "I really don't think he knows that he is being a jerk."
The idea of this as an excuse sort of fills me with rage. Most people, after all, have no problem justifying their behaviors to themselves. Most people do not set out deliberately to cross boundaries or be jackasses. So it is practically the definition of being a jerk to not know, or not be aware, when you are doing something wrong/mean/rude/whatever. One of the jobs of a human being is to actively prevent oneself from being a jerk, which often involves being aware of other people's feelings and perspectives, actively curbing one's natural self-centeredness and inclinations, and so on.
I mean, you know, I'm glad he isn't intentionally evil. But that's not really saying much.