Did I miss something? (Spoilers)
Quite near the end, I think it's the scene just after the police interrogation, when Marianne is walking around the villa calling Paul's name and finds him crouched in the garden, he said something to her that I didn't quite make out. He tells her something, then says, "I tried to save him. I'm so sorry." It sounded to me like he said "I killed him", but her reaction doesn't seem to support that. Then at the very end, after the Marshall pulls them over and she gets back in the car, they sort of laugh in relief and then look at each other very somberly.
So, whether or not he admitted to killing him really changes the tone of the ending. If he did admit it, the look she gives him at the end could be the revelation finally hitting her. Or, if he simply lied again and just said he attempted to save him, that look could be her realizing he must have done it (judging just from his panic at the police pulling them over.) Did anyone catch this? My impression was that Marianne thought he was behind it from the beginning, thus why she immediately asked to call her lawyers in the Marshall's office and why she gave the rather desperate explanation that anyone could have walked up the path to their house from the beach.
Loved the movie, certainly one of my favorites this year. But God did these characters love to whisper sometimes.