Among many things, Sunset Boulevard (1950) tells the story of a love triangle between writer Joe Gillis (William Holden), aging silent film star Norma Desmond (Gloria Swanson), and young script reader Betty Schaefer (Nancy Olson). Norma spends most of the film toying with Joe’s emotions, keeping him within her company as a young, handsome means of convincing herself she’s still a desirable woman. Deluded in her grandeur, she is unable to face the reality of her fading status, and tries desperately to make Joe fall in love with her. In her most desperate moments, she becomes somewhat successful, though Joe’s heart is never fully invested in Norma’s offerings. Meanwhile, Joe has become attached to Betty, a script reader at Paramount. Though she is engaged to Joe’s good friend Artie (Jack Webb), the two initially bond over Joe’s poor writing, buried within which Betty finds good ideas that could amount to a nice story. As the two write a script together, they grow closer, and any attachment Joe had to Norma becomes strictly material.
At the end of the film, just after Joe and Betty admit their feelings for one another, Norma calls Betty on the phone in an attempt to scare her away from Joe. Norma warns Betty about “his type,” and tells Betty that she has no idea where he really lives, or who he really is. Joe walks in and intercepts the call, and tells Betty to come over to Norma’s mansion and see the truth for herself.
When Betty arrives, Joe tells her about his arrangement with Norma. He lets her know he essentially serves as Norma’s live-in lover in exchange for the finer things in life, and tells Betty she’s better off marrying Artie because “he has a good thing going” at Norma’s. It is heartbreaking, as the slowly-building relationship between the two is diminished by Joe himself, shipping Betty off to marry a man she no longer loves.
But immediately following that, Joe goes upstairs and begins to pack his things. He renounces all the gifts Norma has given him -- the cufflinks, the gold cigarette case -- and says he’s leaving her house for good. Not only does he give away Betty, he gives up on Norma, effectively returning to the indebted, lackluster life he had before either of them came into his. Why would he do that?
The broad answer is that Joe has evaluated his situation and doesn’t like the outcomes. He either remains kept by a woman he doesn’t love and tolerates for material gain, or he follows his desires and pursues the woman he loves, breaking up the relationship of a good man in the process. Neither situation has a perfect or overly moral outcome. He chooses to be cruel to Betty by saying he refuses to give up his life of luxury, telling her she’s better off with Artie. It isn’t true, but his feeling is that if she goes away angry at him, her returning to Artie will come easier.
Most of the film sees Joe doing what is easiest for him -- stashing his car to avoid late payments, attempting to use Sheldrake (Fred Clark) for a loan, thoughts of skipping town and heading to Dayton. He’s also a man at the whims of temptation, as evidenced by his encounters with both women. He finds nobility in his decision to return Betty to Artie, and to remove himself as an object of Norma’s lust.
It is the first moment in the story where Joe does what is right as opposed to what he wants. He worked on a bad screenplay just for money, went along with Norma’s daydreams of returning to the screen despite their impossibility, accepts Norma’s gifts despite claiming he doesn’t want them, falls for Artie’s girl even though he sees Artie as a dear friend, lies to Norma about sneaking out of the house at night, and so on. He does what he wants because it’s easier than doing what is right.
Joe's decision to renounce Betty is obviously a difficult one. He has a real connection with her. They have common interests and their chemistry is organic and founded upon mutual experience and tastes. There could easily be a real bond between them. But Joe recognizes he’s not the type of guy she deserves -- that's Artie. The decision is a responsible one, but just like the writing talent he didn’t realize he had until Betty pulled it out of him, it came too late. Betty observed and revealed something within Joe that managed to change him for the better, and lead to his destruction. Joe’s decision to finally make the right, albeit more difficult decision, resulted in his death. Norma couldn’t face the idea of someone leaving her. Her first husband Max (Erich von Stroheim) never did, and he is still around. The husbands that succeeded Max are never fatally noted -- perhaps they tried to leave as Joe did.
The film initially sets Joe and Norma’s relationship with the presumption that Joe plans to take advantage of Norma for all she’s worth. By the end, those roles have been reversed and Joe is the one being used. He becomes degraded to a point he doesn’t realize until Norma calls Betty and warns her about him, by doing little more than stating points of truth. Hearing what she says, he wakes up, discovers she’s right, and decides he can do something to remedy it all. It is the climactic point for Joe’s character -- the cost of which is his life.