Heh I'm going to disagree with all of you and say that he was actually Lawful Good
 This sounds odd right? But there is a very good reason for this which no one seems to have thought of. Basically in England at that time
the law was embodied in the person of the king and while the king lived it was his laws that were right and just. Therefore Robin would have been Lawful in following them and opposing anyone else who decided without the kings authority to set up laws of their own. Well John was not the king...King Richard was still alive and held for ransom but John wouldn't pay the ransom to set him free in order to put forward his own claim to the throne. The sheriff and his cronies where put in power by John (It was often standard procedure for new rulers to put there own supporters in positions of authority) so in Robins view the sheriff had no legitimate authority as his authority was supposed to come from the king and the person who claimed that authority in England was not a legitimate king as the true king was still alive and held captive. Consequently Robin opposed him and his unlawful laws and taxes
 Therefore I don't think there can be any doubt that Robin was lawful and this combined with his care for the poor and oppressed demands that he be Lawful Good