But he is too nervous to get around to being really dangerous.
Alternatively, Bob does become legitimately dangerous... But not in a good way, and there's certainly nothing brave or heroic about it. He simply attempts to slaughter anything or anyone he thinks might be a threat to himself or those he cares about (if he cares about anyone else) whenever he can take them by surprise.
Bob was to afraid to do his duties as a mole, and he only sold out the heroes when he had a perfect chance.