jaelis
Oh this is where the title goes?
I'll put out my own favorite way to handle this kind of thing.
IF you are in a situation where two groups are aware of each other, but neither is expecting a fight to break out, then if one person is in a position to attack without warning and decides to, just let them go first in initiative. (Whether that describes the situation in the OP, I'm not totally sure.)
The whole point of initiative (or any other dice roll in the game) is to resolve random outcomes in an unbiased way. If the DM doesn't think a particular situation should have a random outcome (ie, there's no plausible way the guard should be able to react prior to the attack being made), then don't roll dice.
At the same time, the point of surprise is to reflect what happen when you are attacked when you weren't aware of a threat. Certainly the guards were aware of the party, so they shouldn't be surprised.
(If you were successfully disguised as a guard and decided to attack the other guards, that would make more sense for surprise to happen. Obviously the other guards were still aware of your presence, but they presumably would not consider you a potential threat.)
IF you are in a situation where two groups are aware of each other, but neither is expecting a fight to break out, then if one person is in a position to attack without warning and decides to, just let them go first in initiative. (Whether that describes the situation in the OP, I'm not totally sure.)
The whole point of initiative (or any other dice roll in the game) is to resolve random outcomes in an unbiased way. If the DM doesn't think a particular situation should have a random outcome (ie, there's no plausible way the guard should be able to react prior to the attack being made), then don't roll dice.
At the same time, the point of surprise is to reflect what happen when you are attacked when you weren't aware of a threat. Certainly the guards were aware of the party, so they shouldn't be surprised.
(If you were successfully disguised as a guard and decided to attack the other guards, that would make more sense for surprise to happen. Obviously the other guards were still aware of your presence, but they presumably would not consider you a potential threat.)