Although for most of the novel we are lead to believe that Mugo is a hero, his actions at the end cause readers to question this idea.
Throughout the majority of the novel we see the honorable, noble acts of Mugo: his hunger strike in the British concentration camp, his defense of the pregnant woman being beaten to death. But, wee learn near the end of A Grain of Wheatthat it was, in fact, Mugo who betrayed Kihika and not Karanja. As it turns out this has been the dark secret that Mugo has been harboring for the entire duration of the novel. Mugo was able to save himself from the concentration camp by revealing Kihika as the murderer of District Officer Robson.
Ngugi leaves us to question the exact nature of Mugo's character. His admission at the end certainly helps us to see him in a positive light, but ultimately each reader must make up his or her own mind.