The ending does but a few things, but the things are profound indeed, and the ending does them extraordinarily well.
Rust and Marty's relationship is at last repaired. In retrospect, it could never have been repaired with their sicko quarry still at large. Even coming to terms with Rust's sleeping with Marty's wife, which Marty and Rust sort of do earlier in the episode, would't have been enough. They had to get their monster. And they did with a crucial assist from Marty in recognizing the house in the photograph.
And their conversation, from the front of the hospital to the world beyond, was a thing of wisdom and beauty. Rust confessing the attraction of leaving this world for him, to asking Marty to take him away from the hospital where he might have died, was a little masterpiece in itself. Rust need not die now, because, again, death was not a proper reward for the life he had brought to the world with his putting down of the monster.
And the conversation about the proportions of darkness and light - in the world and in the sky - was a fine, poetic piece of work as well. Marty tries to put in a note of optimism, to cheer Rust up, about stars bringing light to the sky. Rust of course corrects him - the darkness still vastly outweighs the flickering light - but he's energized, anyway, and goes off to whatever the future holds for him. Rust is too deep to recover easily. But he's too deep to die, easily, either. So he'll live. And allow that there may be more light than dark after all.
And so one of the strangest, indelibly memorable detective stories concludes. True Detective will be back next season, they say, with a totally new story and presumably totally new set of major characters. Which is as it should be. Rust and Marty - played to perfection by Matthew McConaughey and Woody Harrelson - have done more than enough, and have earned a true place in television history.
See also True Detective: Socrates in Louisiana
Like philosophic fiction? Try The Plot to Save Socrates ...