The rule is that the characters belong to the player, not the GM. Thanks arkibet. You aren't the target, you just capsulated something I want to address.
Except for the fact that I'm about to disagree, I agree. Games are fun, GMs are the filter for the world around you, and you control your character. The game the characters are in should be about the characters, not the NPCs.
I've seen players do some pretty funky things in x years of gaming (where x is larger than 0, but not actually important to this conversation). Offhand assassinations of important political figures, breaches of the masquerade, giving guff to an otherwise socially-minded dragon... ...I've seen it all.
Many times I've stopped and wondered: What the hell is wrong with Player K? (who was mostly Ken, but I don't want to drop all of this on his shoulders). Many times I've let it slide because the character belonged to the player. And because we're having fun, and fun is what a game is all about.
In retrospect I should have done something. In each case I should have at least said: "Are you sure?" a second time. Because I really didn't understand where the character was coming from. Because I really didn't understand what the Player's goal was. And because in each of the examples above the game effectively came to a screeching and eternal halt within one to two sessions.
The player is the arbiter of the character's motivations. However the player is responsible to the group, and the GM is the best arbiter in this case (unless you like having an ombudsplayer).
If you want your character to get together with the hoopy droogs and after a quick stop at the molochok milk-bar go on a crime-spree oh my brothers, well that's fine by me. Nine times out of ten, there isn't any need for GM intervention of any kind. Consequences? Yes, acts have consequences, and the crime spree may just see your droogs turn on you in a quick fit of ultra-violence. But that's just cause and effect. Character motivation? Understandable.
However there are a multitude of situations where a GM should, even must, question what is going on. A "Why?" or an "Are you sure?" is worth the pound of cure.
Arguments about what the character would or would not do? Not cool, not condusive to fun. But when daggers start getting whipped at Emperors, characters start leaping over traffic while engaged in a running gun-battle, and people start telling the perfectly pleasant and utterly lethal dragon where he can shove his sanctimonious attitude it's time to step up to the plate and start the unpopular side of GMing.
Hmmm. Reading this back I don't see my point, and barely understand what I was trying to say. Bad person, bad GM, and apparently not very intelligent either. Shucks.