In the early part of the story, McMurphy played the system, refusing to get with the program and generally making a complete nuisance of himself. But once he realized that Nurse Ratched has the power to decide as and when he can be released from the institution, Randle started conforming to the rules, much to the disappointment of some of the other patients.
In defying Nurse Ratched for as long as he did, McMurphy effectively let the genie out of the bottle. Once he'd undermined her authority, there was to be no turning back, either for him or for the other men in the institution. For the first time, McMurphy's fellow inmates had been a given an all-too-brief taste of freedom. But now that McMurphy's on his best behavior, whatever freedom they enjoyed has been suddenly snatched away from them.
Lacking the mental strength and maturity to handle such a sudden reversal, Cheswick commits suicide. In turn, this causes McMurphy to reflect on how the other men have come to see him as some kind of hero, a charismatic leader who provides an alternative authority figure to Nurse Ratched. Probably for the first time in his life, McMurphy feels a sense of responsibility toward others. That means standing with the other men and fighting back against the system.