The Justice of a Jury
The Jury system has a rather unusual interpretation in Murder on The Orient Express, at least by Western standards. A self-appointed group of twelve, the same number of people in a jury, convicts Ratchett to death and then murder him. The idea of a "jury" or the Justness of the jury becomes thematic material. The Jury is a symbol of Justness. The Armstrong family justified killing a many because they gathered twelve people together who through that Ratchett should die. However, their idea of a jury is nothing like the courtroom jury or jury as the state intended. They like Poirot, did not rely on any sort of law or otherwise to form their "jury." The "Jury" system is simply a consensus; it puts the responsibility of one man's death on the shoulders of many, rather than one. This is what the state does, the state assigns a jury who decides the fate of a man, but there is control over who is selected to be on the jury. If juries were made up of victims family members the jury would certainly be biased. However, we cannot know for sure that Ratchett did not commit the crime. The novel states that Ratchett, or Cassetti, "gave the law the slip," but he may not have been the man who murdered Daisy Armstrong.
The novel constantly questions what a jury is and how "just," this system of justice is, especially when a jury is self-appointed. The final argument of the novel, consistent with Poirot and all the characters is that Ratchett's murder was "just." The jury they formed, and the consensus of twelve, was right and fair.
The Insufficiency of Law
From talks on Prohibition to murder laws in the United States, law is wholly insufficient in Murder on The Orient Express. Prohibition laws are discussed when Poirot searches Hardman's suitcase for evidence. His suitcase is lined with bottles of liquor and he tells the men that Prohibition hasn't ever "worried me any." Hardman and M. Bouc even discuss the speakeasy (the hidden, illegal bars during prohibition). Hardman is planning on concealing his alcohol by the time he gets to Paris, "what's left over of this little lot will go into a bottle labeled hairwash." Prohibition has not curbed the drinking habits of Hardman.
The insufficiency of US law is exemplified by the fact that Ratchett is able to give US cops, "the slip." By means of enormous wealth and the "secret hold he had over various persons" he was acquitted from the crime. After he was let free, Ratchett (formerly Cassetti) changed his name and went to travel on his money. The book suggests that a murderer in America can go free if he has enough money and connections.
The Morality of Murder
Because Ratchett escapes justice in the United States, the Armstrong family is determined to kill him and prevent him from hurting any more children. One of the main themes of the novel is the morality of murder-is it all right to kill a man, even if law has acquitted him? Is it ever all right to kill a man? The novel suggests, at least by Poirot and the passenger's standards, that murder is Ok under the right circumstances. If the crime is hideous, there are twelve people who agree that a person is truly guilty and that person is still on the loose, and therefore it is fine to kill him. There are obvious emotional costs, most of the servants are in tears throughout the novel, but, overall, the Armstrongs are successful and probably will not receive punishment for their crime.