When Scottie sees his fellow policeman fall to his death, he's disabled not only from acrophobia, but also by a crushing sense of guilt. Even if he doesn't say so explicitly, we sense that Scottie believes that he caused the officer's death because the officer was trying to save his life. Getting involved with saving Madeleine may have been a way to get rid of some of that guilt. When he thinks his fear of heights also killed Madeleine, he loses it completely and ends up in a sanitarium suffering from "acute melancholia together with a guilt complex." It takes him a long time to recover.
Once he's released, his guilt seems to send him on a compulsive quest to rediscover "Madeleine" in the person of another woman, and to obsessively remake Judy into his lost love. Maybe he thinks he can symbolically undo her death and overcome his guilt. Instead, he indirectly causes Judy's death in the process of trying to re-enact the scene of Madeleine's death. In the last scene, we see him standing on the ledge staring down and we can only imagine what the guilt will do to him this time.
Questions About Guilt
Is guilt or lost love the cause of Scottie's mental breakdown and hospitalization?
What does Judy's guilt drive her to do?
Do you think Hitchcock is really interested in whether Scottie is responsible for Judy's death?
Does the coroner's investigation do anything to assuage Scottie's sense of guilt?
Chew on This
Scottie never really feels guilt about the worst thing he does: forcing Judy to become Madeleine.
Scottie's also indirectly responsible for the death of the real Madeleine, since he goes along with a scheme that allows Elster to murder her.