Locke's been unhinged from day one, really. He's been hiding information ever since the hatch was found, and we don't know why. He seems to believe in the island itself as some sort of deity - one which appears to need some sort of... sacrifice?... to give its healing powers to him.

It's the information that these people hide that will kill them; when they act cooperatively, they manage fairly well.

Off the top of my head, Sawyer, Locke, Sayid, Jack, and Kate knew about the gun case.