What is the main idea of Murder on the Orient Express?

The main point that Agatha Christie is raising is the difference between justice and revenge. Although the perpetrators feel justified in their actions, they have taken the law into their own hands and acted to avenge the crimes that Ratchett (or Cassetti) committed. They deliberately chose the train because they could congregate there without drawing attention and because they would be moving across national boundaries. Because they believed that the legal process that should have brought Cassetti to justice had failed, the twelve killers functioned as judge, jury, and executioner. Their action was murder. If Hercule Poirot had not been on the train, perhaps they could have succeeded in keeping it a secret. Poirot—like the reader—must wrestle with his conscience and decide if they should be turned in or if his alternate theory should be presented.