I agree with Spencer that you were right in stopping. I too would want someone to assist my wife or daughters, however, as previously stated by others it's best to try and defuse a situation like this than to accept a challenge. The gentleman gave you a forwarning of his mental state by his actions you observed. The safety of the woman and child as well as yourself should be primary to any ego contest sought by the man. Trust me, I've been there numerous times, resisting the challenge and maintaining a calming presence a very hard. If you have a cellphone, next time call the police before even getting out of the car.