Escape
He was going to kill Locke the next time he saw him.
They were about to escape! He was about to get away from this horrible, accursed Island, make it to some civilized country, and get all his people rescued. Locke had ruined the opportunity for everyone. And why? Because he had some kind of emotional dependence on the Island that he translated into a nearly religious fervor. It would be fine if he wanted to stay, but like all religious people he couldn't keep it to himself.
So much for the Hippocratic Oath. He was going to kill him.