If I had to bet, I would say that most jerks in--let's say WoW--aren't really bad people in real life. I do think that maybe there are issues they deal with like low self-esteem, lack of confidence, not fitting in, socially awkward, and things like that. But bad? I wouldn't say so.
But in an anonymous setting like WoW, with all of the powers and freedoms it offers, I believe it gives these people a chance to be on top for once. And, unused to such success in real life, it goes a bit to their heads.
The main factor in all of this is the anonymity a game provides, combined with the lack of repercussions. I say it's the main factor because I discovered a trick to dealing with online dirtbags in Halo 2 & 3 that also works nicely in WoW. The trick is thus:
1. An online exchange of text or voice goes south, and the other player is saying things like, "I'll kick your ass", "You suck"--and all the variations", etc.
2. If something clever to say presents itself, I use it. This has a 40% chance of ending the harassment. (cause it's usually funny).
3. If that doesn't work, I feed straight lines to pump up the vitriol of the agressor.
4. I "let myself get mad", and start "losing control". Basically appearing to sink down to the agressors level.
5. When the agressor has reached his peak, I change my persona to one of focused calm, with an almost pleasant tone--except making it obvious that something is off. Regardless of how the agressor reacts to this, I begin the questions:
"Where do you live?" -this always gets a snide remark.
"Seriously, what city are you in? Come on, tell me the State at least." -still snide, but the wheels start to turn.
"What's your address? Fine, what's your phone number? I can do a search." -the agressor begins to back down and goes on the defensive, "I, I'm not telling you," etc.
"What's your zip code? What pets do you have? What's your Mom's license plate number? What color is your house?" -these odd but scary questions make them think harder, and see danger in saying anything at all. But since the questions are odd, they usually try to regroup in a half-hearted and futile manner.
"Hey! Tell me your social security number now! What is your social security number?!" -the agressor is now in full defense mode, and are only looking for "flight" options and not "fight" ones. At this point, they either become conciliatory or disappear.
The reason this trick works--obviously--is that the tough, strong, anonymous jerk suddenly finds himself in what is apparently a real world situation with real world consequences. It's unnerving, throws them off-balance, and makes being an ass not fun anymore.
I'm sure this wouldn't work for everybody all the time, but it has worked every time for me across three console games and WoW. Maybe this is because I've only ever had the opportunity to use it on teens and early-twenties kids. Maybe it wouldn't work if the agressor was a true badass in real life.
Then again, a true badass wouldn't feel the need to play out any insecurities in an online game.