If he were not suited for duty, why was he back on the ship in the first place? He's already abandoned his executive officer on an icy planet and then attempted to kill him when he found a way back on board. If he's so close to the edge he'd have no place being on the ship in the first place. It's not like he was needed on the ship. Kirk was all ready to depart when Spock showed up and offered his services. I wonder who the first officer was that he pushed aside was?