I think that's a lot of it, but I've seen my share of flamewars between people who have developed online "identities" (e.g. active long-time members of Slashdot).  Maybe these "identities" aren't robust enough, or maybe these people really are loud-mouthed jerks in real life.  Maybe when alyssa says (below) that it's always harder to flame people in real life, that's another way of saying that online identities can never be sufficient.

A part, but then again everyone's part asshole.  What part of their nature someone chooses to repress is, I think, perhaps more important than their basic impulses.  You can test the combination of impulses and repression either offline or online.  I prefer to think offline is the truer test, both because we've evolved to deal with it, and because the alternative is just too depressing.