This film is concerned with the ideas of guilt and justice. Hitchcock's films commonly, if not always, revolved around crimes. His signature is to ensure that the criminals receive just punishment by the end of the film, even if they are allowed to get away with their crimes in the eyes of the law. Their punishment, then, often looks like emotional disturbance.
In Dial M for Murder, both Halliday and Swann face judgement for their crimes but in different ways. Tony is forced to repeatedly evolve the elaborate lie which he invented around Swann's death. He wants his wife dead, but more importantly he doesn't want to lose his to accomplish that. When Margot kills Swann in self-defense, Tony seizes the opportunity to frame her for murder and thus eliminate both of his problems. Unfortunately for him, he isn't clever enough to outwit both the police and Mark Halliday. In the end, he is discovered for all the lying and scheming and convicted in a court of law. On the other hand, Swann faces a more direct and retributive justice for his crime when Margot stabs him to death. He is given exactly what he intended to deliver. Nature had her way with him.
When all is said and done, the film concludes with what appears to be justice delivered all around. The tone, however, is not celebratory. This is where the discussion of guilt enters the picture. While Margot certainly didn't deserve to be murdered by her husband, she wasn't a saint either. She was carrying on an affair with her husband's friend, Mark. She was not an innocent victim in the story because she was able to be blackmailed in the first place. In much the same way, Swann isn't a victim of Tony's manipulation because he had such a sordid past that he was easily blackmailed into committing murder. If Swann hadn't led such an indiscriminate, criminal past, he could've easily turned Tony over to the authorities for plotting to murder his wife. As it played out, though, each character, except for the somewhat innocent Mark, made wrong choices which opened themselves up to a world of hurt and retributive justice.
