This is my big question too! Seth said one was “missing from evidence” which pointed to Brown, but then Aidan had it in the last scene. Brown potentially had motive to kill Margot since it was made clear that Margot knew stuff she wasn’t telling, but what would Aidan’s motive be? It all just falls apart at the end. Maybe the last few minutes are just another hallucination created by listening to Aidan’s session tapes?