I had one who just was apparently inconvenienced by everything and everyone. Plops in, takes off shoes (no socks). Every time anyone got even close to him while boarding he'd glare, then mutter some comment about people can't walk straight. Fussed at the FA because he had to ask twice (within a 45 second period) for a drink while people were boarding.

My favorite though, after we're airborne, and long before the announcement, grabs his laptop and sets up to start whatever critically important work he had to do. Then the guy in front of him reclined. Now, the seatback came nowhere near the laptop, but the guy started hitting the seatback. Didn't say anything, just kept pushing it until the guy in front of him sat back up (after a few glares). Then he had the gall to say, loud enough for people to hear "Some people just have no courtesy".