Full Metal Jacket has equilibria on a more somber note than what some people might expect, but that's only because of the nature of the climax in each half. Just as the story of Oedipus ends in tragedy due to his negative self-revelation, Joker's story in Full Metal Jacket reflects how he changes by the end.
In the Parris Island segment, he has completed training but is still passive enough that Pyle kills Hartman. As such, Joker completes recruit training with uncertainty, knowing that he's not fully prepared for the life of a Marine. Later, in Vietnam, he accepts the role of violence in his profession, cemented by his killing of the sniper.
Joker completes his tale with one of the iconic lines of the film: "I'm in a world of shit, yes, but I'm alive."