So, I just listened to this in flashback EP 767, and seeing as I couldn't find a thread for the flashback, I decided to comment here. I really enjoyed this story. As Unblinking said more than 10 years ago (!!!), it is rich with details that kept you hooked.
I am not completely sure about the ending. I interpret it to mean that he will use the mirror to kill himself rather than the president. I also didn't connect the mention of the president with any particular president (living or dead) and I didn't have any more issue with him being asked to kill the US president than I had with him being asked to kill the Canadian Prime Minister. Both are assassinations of a head of state, and so equally horrible. I think those were specifically mentioned to build up the horror of the character's situation, so that we agree with his decision to use the mirror to kill himself and stop all of this.