Why does the narrator now understand why he never felt any guilt for his actions and thoughts before?

With "I" newly in his vocabulary, his internal conflict and search for a replacement to collectivism have resolved, reconciling his instinct with his consciousness. His society used words to restrain him, but now he uses words to free himself from collectivist doctrine, and he recognizes that he made an error when he sought a reason for existence. His search has in one sense culminated in a word, but in the broader sense, it has culminated in his self.