Obviously the best option would be to let them kill each other and just redo the whole thing. Imo the most sus is the black dragon guild master, she wasn't taking part in the fight, which could indicate that she was waiting for the rest to kill each other. Also she was probably the one behind saintess trying to poison the "fire retard", because it likely wasn't the first attempt, since he asked specifically about her being sent by black dragon guild master. Although merchant lady was idle too during the fight, she is unlikely to be the traitor imo. Inquisitor might be the impostor, but I think that would be too generic, also too much involvement in the fight.