4 ideas
Samuel Ratchett, a wealthy American found stabbed twelve times in his compartment on the snowbound Orient Express, is revealed to be Cassetti, the gangster who kidnapped and murdered three-year-old Daisy Armstrong and escaped conviction through money and technicalities. Poirot works out the truth and then gives the police a false solution, an unknown intruder, letting the conspirators go free.
When strangers who supposedly have no connection vouch for one another's whereabouts in pairs that interlock too neatly, the fit itself is evidence of coordination. Independent witnesses produce testimony with gaps, contradictions and irrelevant noise. A web of alibis with none of that friction points to a single designer rather than to innocence.
A crime scene holding too many contradictory clues, such as a handkerchief, a pipe cleaner, a uniform button and a stopped watch, should be read as a set of messages written to be found rather than traces left by accident. The investigator's question changes from 'what does this clue prove?' to 'who benefits from my believing it, and why would the real culprit leave it?' Excess evidence is itself a fact about the crime.
Once the formal legal system has failed to punish a proven murderer, private collective retribution can be morally defensible even though it remains legally a crime. Poirot's decision to offer the authorities a false explanation treats the finder of truth as free to choose which truth to report, putting moral judgment above procedure.