Why does McMurphy say that the other patients in the ward have “conned ol’ R. P. McMurphy”?

McMurphy is referring to his conflict with Nurse Ratched. He has knowingly taken on the other patients' fight against her injustice, but he has just come to the conclusion that they conned him into protecting and standing up for them. Up until this point, McMurphy didn't know that Ratched held the power of keeping him in the ward or having him released.

I couldn’t figure it at first, why you guys were coming to me like I was some kind of savior. Then I just happened to find out about the way the nurses have the big say as to who gets discharged and who doesn’t. And I got wise awful damned fast. I said, ‘Why, those slippery bastards have conned me, snowed me into holding their bag. If that don’t beat all, conned ol’ R. P. McMurphy,’…Well I don’t mean nothing personal, you understand, buddies, but screw that noise. I want out of here just as much as the rest of you. I got just as much to lose hassling that old buzzard as you do.