At the end of One Flew Over the Cuckoo's Nest by Ken Kesey, the tyrannical Nurse Ratched has R.P. McMurphy lobotomized. Chief Bromden decides to suffocate his friend and make his escape.

Is Bromden justified? Opinions may vary from reader to reader. Let's consider why Bromden acts the way he does. McMurphy spends most of the novel attempting to undermine Nurse Ratched's rule. When she has him lobotomized, which leaves him in a vegetative state, it appears that she has "won." Chief Bromden believes suffocating McMurphy is an act of mercy, saving him from a comatose life forever on the ward under Nurse Ratched's rule. Chief's mercy killing is his way of helping McMurphy reach freedom—and Chief himself seeks freedom by smashing the control panel through the window and running off. Chief Bromden believes this is his only choice, and many readers would agree that his actions are justified. One quote to support this is:

I lifted the pillow, and in the moonlight I saw the expression hadn't changed from the blank, dead-end look the least bit, even under suffocation. I took my thumbs and pushed the lids down and held them till they stayed.

The unchanging expression implies that McMurphy was already gone. His mind was gone, and only his body was left. By closing McMurphy's eyes, Chief shows that he cares for McMurphy, and his motivations were out of love.

Does Chief Bromden have any alternatives? Well, he could escape without touching McMurphy. However, this would leave McMurphy's body under the rule of Nurse Ratched. McMurphy makes it clear that he does not want to remain on the ward. So if Chief wants to help his friend, this is his only choice.

Chief Bromden could also choose to not escape, but what would that achieve? Classified as a chronic, Chief is committed and therefore is unable to check out or transfer himself. He would be stuck suffering on the ward. Is that a life worth living? Some people would say no, which is why they would support Chief's actions.

As this is an opinion-based question, yours might vary. However, using evidence to support your answer will always be useful.

The Chief felt that McMurphy was gone and he was technically correct in this; lobotomization has serious effects on personality and functionality (most patients are incapable of living independently afterward). In consideration of McMurphy as the representation of life, freedom, and the opposite of the oppressed world in which Bromden lives, it is easy to assume there were no other options. In his way, Bromden was "freeing" McMurphy from the mental prison in which the lobotomy had ensnared him. This action also "freed" the Chief, giving him few personal options but to leave. 

In terms of the novel's intentions, Bromden had no choice but to suffocate McMurphy; Ratched's objective in having McMurphy lobotomized was to provide a warning to the patients of the ward--"Don't mess with me..." In killing McMurphy, Bromden made him a martyr for their cause, a symbol of resistance who died in battle.