As in the DC Comics, Batman villain of the same name.
That horrible halfway point between Loonie and Power Gamer.
In PC terms they are about as close as you can be to the "Boss" of Saints Row with out falling into pure camp. (And some do fall into camp.)
What I'm talking about is a type of player, and player behavior, that is THE most destructive to the enjoyment of a game outside of real life drama. It's the player who has no investment in the game or the setting. They are willing to ruin other players fun just because they can. A lapse of compassion devolves into the worst kind of Solipsism. This goes WORLDS beyond my Grand Theft Morality. Welcome to a player character psychotic break.
So what causes it?
Well, in the case of usually good players it could be boredom mixed with a lack of consideration for the rest of the group. If suddenly a normally rational player starts murdering NPCS, or botching missions with lame excuses as to WHY, then it might be time to pause the game and ask the player what is wrong. Often they didn't consider the long term impact of their actions might ruin the fun for others. Worse if the behavior is started by a group of players, say 2-3, and it derails the game in ways that don't seem logical. This campaign hijacking might be claimed to be "in character", but honestly if the character is that caustic to the idea of going along with the rest of the group...why are they even there? Again, stop the game and ask the player whats going on.
Those are the more typical, reasonable issues where a little GM tending of your flock can fix most things.
Then you get the jerks, the pure numbskulls who just like causing chaos in games.
The BIG difference with these guys is it's habitual. You'll have a few cases of 'wtf'ery followed by hollow excuses. Then the next session it happens again, and it escalates. You can tell it's happening because the problem player is testing you, the GMs, boundaries. Each time upping the ante on just what they're trying to get away with. NPC murder, and theft, mission derailment, inner party bickering, etc, etc. And it just keeps happening.
That alone should have caused you as a GM to pause and ask them what's going on. And odds are they said, "Sorry, just having fun. I'll tone it back." and then they don't. Congrats, you have the worse kind of problem player. These types are rare, since this kind of griefing is easier to pull off on FPS games or MMOs and they get a faster 'buzz' of trolling another group. The real monsters are the ones who want a challenge and seek out groups of players to ruin and troll the player.
There really isn't any way to redeem these types, I'd suggest you just kick them from your game. It may be painful if they are friends with other players, but DON'T PUT UP WITH IT. You wouldn't want them around if you were playing chess and they kept knocking the pieces over, or you wouldn't want them there to watch a movie if they kept spoiling the plot or talking in the good parts. Same thing here, they are ruining the fun for the group.
Now I've heard that some younger players don't know that the behavior is mean, and they might be turned around. But at some point you have to draw a mental line. If the player keeps crossing it, call it.
The Joker isn't there to make you laugh, he's not there for your fun. He just wants to see you suffer, and your game goes up in flames. Just for the luz.
Batman had it wrong. The best way to beat the Joker? Laugh at him and stop playing his game.