If the articles and comments this week are any indication, there are N definitions of the word "griefer," where N is the number of people in any given conversation. So let me get us all on the same page by giving my definition of the word. It's okay if your definition is different, just don't confuse it with the one I'm using between here and the byline.
Note that sometimes the name "griefer" is given to someone who is simply a competitive player who doesn't have an outlet for his desire to compete. If he's looking for PvP action in a strongly cooperative game, then he's likely to get bored or frustrated at some point. Enough with this "working together" crap! I want to know which one of us can kick the most ass and I'm willing to bet it's me! The game isn't letting him test himself against other players, which is what he really looks for in a game. Rather than doing the grown-up thing and finding a game more to his tastes, he tries to bend the current game to his style of play. These players are annoying and often cause grief for other people, but they are not griefers in my own nomenclature of online annoyances. They're just jerks. They're mostly people who are in the wrong game (or on the wrong server) and they're usually capable of playing properly if they can find the kind of gameplay that suits them.
In real life, running up the score when you're already ahead is not griefing in my book. But it is griefing if you hide the ball or kick it out of bounds over and over so that the game can't progress. If two junior football teams are wildly mismatched, it's not griefing. Playing poorly because of lack of skill or experience is not griefing. Hurting your teammates or playing in such a way as to make your team lose on purpose is griefing. Bad sportsmanship is not griefing. Even cheating is not necessarily griefing, assuming the cheater is simply breaking the rules in order to win.
No, the griefing I'm talking about is when one person stops trying to play the game and starts trying to stop other people from having fun, by any means necessary. Their goal is not to win the game, but to make other people miserable. This is my definition of griefer.
In real life, we just call them bullies.
Bullying is not a new or unknown quantity in human behavior. Some people hurt, or feel weak, or have this need to share their pain and frustration with others. They are not hurting others because they want to take something, or because they want to force someone to their will. For bullies, hurting others isn't a means to an end, it is an end in itself. Bullies often form chains of misery, with one abused person finding someone weaker than they are to bear the brunt of their frustrations, who in turn finds someone else, yea, even unto the seventh generation. It's like the circle of life in The Lion King, except this is the Circle of Assholes. In the real world, bullies form a hierarchy based on who is the strongest, but online the bullies are organized by who knows the most exploits and has the most free time.
The in-game griefer usually points the finger of blame at the victim for "sucking so bad," but that defense makes about as much sense as a rapist who says he's innocent because his victim wasn't any good at fighting back. More reasonable people might point the finger of blame at the griefer, but I think they're both wrong. In an online game, the guilty party is the one who allowed the griefing to happen in the first place. If you want to be mad at someone, be mad at the developer.