Bob is the weakest or least talented member of his group, and he knows it. As a result, he prefers to let the stronger members handle problems and stay on the sidelines.
Bob has had numerous bad experiences to the point that he views bravery as practically suicidal and views his own cowardice as plain good sense. He considers himself a pragmatist, not a coward.
Bob was forcibly dragged along against his will by the rest of the group, despite his constant and repeated protests that he couldn't help them. Knowing that he's in way over his head, and feeling no loyalty to the idiots who got him into this mess in the first place, Bob cuts out and abandons them at the first chance he gets.
Bob is a Fearless Fool who attacks the enemy even when running away would actually be a better idea.
Bob's lack of fear leads him to do some pretty heinous things as he does not care for the consequences of his actions.
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.