Why did the vampires in the theater laugh at Louis?
Louis is the “gentleman” of the vampires and the most human, unlike the vampires in the theater. When the boy (Dennis) made a sound of pain, Louis stopped because he didn’t want to hurt him. He also didn’t want to open the same wound twice. Viewers have variously interpreted the vampires’ laughs during this scene as being because they found amusement at the boy’s pain or at Louis’ attempts to not cause pain.