I'm a major dick all around, at least as far as I can tell. I do TONS of shit for my own enjoyment. Like in the situation you described, I'd adjust my speed accordingly so that you couldn't get over (assuming I was at the front of the line where you needed to merge, etc). Not to be mean, or "teach you a lesson," but just because I think it's funny, because I know the other person is raging at me.
I do lots of stuff for the lulz. My office is usually pretty laid back, because we're so far from the main headquarters, so I brought up a ton of little piddly shit that no one here cares about, but is a major HR violation, just to watch the anarchy. Oh god it was glorious. People were wondering who had turned them in. They were bitter at HR, wanting them to leave us alone and let things go back to the way they were. It's been almost 6 months now, and it still isn't
Yeah, I'm THAT guy.
Edit: Holy damn I love commas