My theory for the a-hole phenomenon is lack of maturity, lack of self-control/self-discipline, and lack of home-training(as we old-schoolers used to call it). Also possibly a lack of ability to separate fantasy from reality and taking a game way too seriously, but maybe I'm being too harsh in my criticism.
As pissed as I've gotten at other players in MMOs, I find it inaccurate to generalize any type of "hostile" behavior as being based in some serious personal deficiencies on the player's part. Mainly because in most MMOs, and especially WoW, it's really too easy to be a dick, and in the end, there really aren't any repercussions.
I mean, say you are on a PvP server (which is the only place you can actually get ganked). Well, just by being on a PvP server you know that you are going to get killed...a lot. Especially when it's so easy for higher level characters to kill you. Click one button on a sufficiently lower level player and they are dead. 30 seconds later they are back at their corpse and back to playing. Plus, people seem to go crazy with the about of meta-rules they think people should follow in PvP encounters in-game. I swear some people won't be satisfied unless all PvP encounters are /duel with 5 minutes to prep themselves before any fight starts.
I played a rogue in WoW. I've done my share of ganking and getting ganked(by other rogues, no less) in PvP, as well. Yes, it does piss you off, until you realize that it's all part of the game. Once you realize that, you calm down and just enjoy playing the game, rather than tearing off on a childish tantrum of rage.
But, I'm not referring to the people who get occasionally angry at events in the game(I realize that's not entirely clear in my post, and everyone gets angry from time to time). I'm talking about the people who repeatedly go into the childish fits of rage, throwing things, yelling and screaming. I'm talking about people who destroy their keyboards and mice in their rage, or scream at everyone on vent or teamspeak. I'm talking about the people who will expend all their energies typing insults and obscenities in chat, directed at their team, rather than focus on playing the game, or just leaving if they don't like where things are going.
Then, there are the people who make it their life's passion to persistently grief other players(the PKer type) in such a manner to cause the other player time and effort lost. This isn't just normal ganking. This person is hunting you, targeting you, specifically, seeking the right moment to make all your efforts be for naught. Or that person who constantly attacks you, and you alone, out of all other possibilities; he will actually pass up all other targets simply to enjoy killing you, specifically, again and again and again. I actually ran across a posting by a PKer who mentions that his entire joy in the game is making others lose the time and effort they've put into the game; in fact, here's the link http://www.karashur.net/mmorpg-articles/wowvsl2.php. While he has some interesting, and even good, suggestions at the end, the overall attitude I perceived in his post, in my opinion, is not one conducive to fun play for larger population of players, even in a normal PvP context.
Then there are those people who actually steal kills and resource taps. I'm not talking about an accidental convergence of two players on the same target(that happens and it's usually evident when it does). No, I'm talking about the person that will actually come over and try to take what is obviously yours. Not only that, but then that same person will follow you looking for another opportunity to perform the same act. While this is not the same as PKing, it is a form of deliberate and targeted griefing.
Then there are those who allow their anger in the game to bleed into real-life and project that anger onto innocents(i.e. other players) who may not have been directly involved in the acts committed against them. These people have no real knowledge of the other person; yet, they stereotype the person immediately and shun them because they play a particular faction or character type(I played a rogue exclusively, which is nothing but a piss off to everyone else). That's just not a good way of playing a game or dealing with other players of the game. It is, after all, just a game.
These are the types to which I am referring in my original post. In my opinion, these kinds of actions from such people constitute bad sportsmanship, even in a PvP context, and it can make even PvP become not fun.
There is also the problem that you are dealing with real people in a gaming context, and whenever you deal with people, whatever the context, social rules and etiquette become applicable.
**EDIT: Minor grammatical corrections.