How can anyone be an asshole on purpose, when I feel so lousy when I do it by accident?
It depends on the target of the assholishness, for me. If I'm antagonizing a very bad person, I'm doing something good, or at least something excusable. If I'm fighting back against said person after an attack, even better. If I'm being an asshole in a joking manner to friends, that's fine if they can laugh about it.
It's really down to intention, and to whether the person deserves some pain.