During the level Undercover, there is a way to kill all of the Hammerites without causing any commotion. Find a hammer in one of the rooms. When you see a Hammerite and no one else is around, throw the hammer at them a few times until they fall over and die. Even as you are in the process of killing of them, they will not get mad. Then, throw the body in a random room where no one will find it. You can now go in all the forbidden rooms and steal all the gold desired without getting caught.