Chief lies in bed at night and hears some footsteps approaching. He sits up and realizes that two orderlies have brought McMurphy back to the ward. He seems to need assistance to lie back down.
After the orderlies have left, Chief goes over to check out McMurphy. He says that he heard McMurphy had escaped, but knew McMurphy wouldn't leave without him. Chief was waiting for him, but now it's time to go.
Chief realizes that something is wrong.
He inspects McMurphy's head and sees two large scars on his forehead. That means that hospital has given him a frontal lobotomy, which means that McMurphy is basically a zombie for the rest of his life. He can't even speak anymore.
Chief hugs McMurphy to his chest. He says he's not going to leave McMurphy the way he is.
So he grabs a pillow and smothers McMurphy, killing him.
Once McMurphy is dead, Chief walks over to the water fountain (remember the one McMurphy couldn't lift).
He tears it out of the floor and uses it to punch a hole in the window screen. Another patient (Taber) wakes up and shouts in triumph as Chief jumps out the window and runs off toward a nearby forest.