SPOILERS: The key to get Swann in

If I was Tony planning a perfect murder, why didn't he just make a copy of the key and give it to Swann to get in? Therefore there would be no need to steal the key from Margot's purse before he left. Her key would always be in her purse.

If the plan worked, then Swann would have entered the apartment with the copied key, killed her as planned, left a bunch of things in the suitcase to show it was a break-in and still had both his own and the copied key, and walked out the door. And Margot is dead on the floor. How sad. Through Mark's testimony, the police would have known that her purse was stolen and she was being blackmailed and the purse thief could have made his own copy to get in.

And then, when the plan didn't work out, Tony would have come home, found two keys on Swann's body, and figured out which was the key to the apartment and taken it (and hid it or something). Tony still could have planted the letter on Swann, the police would assume Margot let him in, and Margot would be hanged.

Sounds simple enough, and a more perfect murder to me.