The entire film dealt with his guilt and frustration, the mistakes that almost destroyed his marriage. He was weak and fell victim to his temptations. However, he never knew that Carolyn was killed precisely because of his mistake. Until this final scene, he realizes that it was his fault. Had he not slept with Carolyn, his wife would have never committed the murder.
This final scene in the film is shot with great simplicity, but also with great effectiveness. Director Alan J. Pakula uses only two shots at a time. First he begins with medium shots of each actor. As the scene (and revelations) become more intense, he comes closer and uses only close-ups for the rest of the scene. The close-ups make the scene a lot more personal and emotional. The close-ups also box the characters in. Rusty can’t do anything but listen to his wife confess. Harrison Ford even contributes by shedding some tears. Nothing but their emotions and pain are seen in this scene. And above all, Rusty’s guilt is truly felt because of it, helping conclude one of the main themes of the film.