Here's my Top Ten....................
1. And Then There Were None
2.Murder on the Orient Express
3. Murder of Roger Ackroyd
4. Cards on the Table
5. A Murder Is Announced
6. Crooked House
7. The ABC Murders
10. Death on the Nile
I really wanted to add The Tuesday Club Murders - I know that it's short stories but the great framing device of having them meet each week and take turns made it seem like one big book to me.
The Mystery Man was N.K. Reporter.
His motive was to protect his job and promote his career to international status.
I think that he had discovered through the journalistic grapevine, or possibly Jessica had discovered and had told him, that the international news conglomerate was looking for small newspapers to add to its stable. He had to find a way to bring the Gazette to the attention of the conglomerate. He also knew that Jessica was the better reporter. (I concluded this because the spelling errors appeared only in his articles and not Jessica's, and it was later borne out by the fact that it was Jessica who became "the international face of the Gazette".) If the takeover happened he knew that his job would be in jeopardy.
His primary objective was to kill Jessica, but he needed to camouflage this fact and decided to make it look as if a serial murderer was at work. This would also serve as a good way to bring the Gazette to international attention. Rather than choose his victims at random, he hit on the idea of killing people who had committed a crime in the past. In the minds of the public and the police, this would appear to be a kind of justification for the murders and it could also create the impression that the victims had a common link, which actually they did not have, except that they all lived in St. Mary Mead and had all had their palms read at the village fete.
As a reporter he had access to information in police and court records, newspaper archives and so on and this enabled him to compile his list of victims.
Now we come to Mystic Margot, and this is where I come unstuck. The Mystery Man must have approached her before the village fete and somehow persuaded her to tell everyone on his list that she foresaw their murders. He succeeded in getting her to do it but for the life of me I can't see why she agreed. There doesn't seem to have been any benefit to her other than a bit of publicity. Her later statement that "I don't know what is truth or what is fiction any more" seems to indicate that she believed some of the future events would be reported honestly and others not, or possibly she was told that only some people on the list would actually be killed and not others, based on the seriousness of their crimes, perhaps. Even if the whole enterprise was presented to her as an elaborate hoax, I still can't see what was in it for her. There are only two possibilities that I can think of. Either Mystery Man was able to blackmail her into taking part, or she did it because she believed it would help someone who was extremely important to her. I can't find anything to support either of those theories though and I couldn't come up with another theory that explained everything else.
The only link I found between Margot and any of the other characters came from some external research which informed me that the name Greer means "watchful" or "vigilant" and the name Jessica means "watchful" or "foresight". It's a tenuous link at best, and requires one to suspend one's disbelief in the occult, but perhaps Jessica and Margot were blood relatives (sharing the gift of second sight etc.) and Margot thought she would be helping Jessica in her career. Even though Agatha Christie did write seriously about the occult, I don't like this as a theory and I don't like using something that wasn't presented to us in the game.
So there you are, I considered myself checkmated by the compilers and did not submit an entry. My congratulations to them and many thanks for a most enjoyable game. Roll on next year!