I have played with some really great and some really bad gamers over the years, but my all time favorite groups have been the one(s) that completely buck the system. The groups that seem to live by their own warped moral code. It goes deeper than hack and slash or running evil aligned PCs, though on the surface that is exactly how they seemed.
I know that as DM/GM I can control this behavior in players by having repercussions in game, and I have done this. During these "good" games the players did stay in character, and did a damn fine job of it, but their hearts weren't in it. For some reason they always wanted to play bad characters that sometimes did good things. They would steal from each other, undermine each other, attack and sometimes kill each other, and they liked playing that way. For some reason they could come up with better reasons for being a party with these types of characters than with a group that is suppose to be doing "good". It always seemed like they were more of a gang than an adventuring party.
I can't tell you how many times characters would openly "go after" another PC, spending an entire nights game trying to "do them in", whether it was by spreading unfavorable rumors, stealing possessions or just cutting their heads off. Games would sometimes go like this:
DM: "Your group is sitting in the darkened corner of the tavern, farthest from the door. The barmaid has just brought you another round of drink as the door to the tavern busrts open. You see a man dressed in the garb of the city watch standing there clutching his right side. Even from where you are sitting you can see the fresh blood staining his clothes and dripping off his hand, that is covering his wound."
PC1: "How close am I sitting to PC2?"
DM: "About 3 feet."
PC1: "I want to take the change purse he tied to his belt before we came in."
Check are rolled and PC 2 feels PC 1 trying to lift his purse.
PC2: "I pretend that I don't know what is going on and put my crossbow, which if you remember I loaded before we came in, in my lap and aim it at PC1."
PC1: "Do I see him to that?"
More checks are rolled and PC1 fails his checks.
PC2: "I shoot PC1."
A fight breaks out, PC2 & 3 kill PC1, while PC 4 uses the multiple distractions to steal stuff rom around the tavern.
Like I said before, I did control the situation on numerous occassions, but it ruined the game, so I eventually quit doing it. In some ways this made me a better DM/GM as I had to learn to be very adaptive, make my adventure work in the context and spirit of the game and most importantly to improvise. I was resistant at first, but actually came to appreciate their style of play.
Despite all the infighting and subterfuge, when the chips were down most of the time they were one hell of a team. Mostly when the enemy was external and they had something to gain. While it may seem that groups like this are a royal pain, they can be really interesting and rewarding in their own way. Gaming like this does limit the type of adventures and you sometimes have to railroad playes, but if you are a halfway descent DM you can make it seem as if it what you want is a natural result of what the PCs have done in game. Of course in groups like this it helps if the everyone is good friends and understands that this is just a game, and don't take it personally. This includes the DM, who sometimes gets upset (I use to before I came around to their way of thinking) when the countless hours he/she spent planning a session and creating "brilliant" plot hooks gets ignored. While it is all in good fun, it does suck that most of the PCs don't get to see very high levels and sometimes your well planned adventures turn out completely different and often better than you had originally planned.