It seems that in MGS4, Liquid has fully taken Ocelots body, because his arm is free on it and he i swearing Liquids clothes on MGS4 artwork. He was using the arm to get his way into the body. The thing about Big Boss 'turn' i think, is that the thing which happened to him in operation Snake Eater, the thing about the Boss, and it mabye made him mad over time and after the 'les enfant teribles' project, he may of thought that he was just a tool been used, so he decided to make his own rebbelion, and the Americans turned against him, sending his own son to kill him!!