"Cassetti was the man! But by means of the enormous wealth he had piled up and by the secret hold he had over various persons, he was acquitted on some technical inaccuracy. Notwithstanding that, he would have been lynched by the populace had he not been clever enough to give them the slip." 

In Part One, Chapter 8, Poirot describes the details of the Armstrong kidnapping case as well as the unethical means through which Ratchett, or Cassetti, escaped punishment for his crimes. The fact that he was able to avoid facing any consequences for the brutal kidnapping and killing of Daisy Armstrong due to a “technical inaccuracy” emphasizes the inability of the law to always ensure a just outcome. Mentions of the “secret hold” he had over others as a result of his wealth also serves as a critique of those with the responsibility of carrying out the law, hinting at their corruptibility. 

“‘Then in my opinion the swine deserved what he got. Though I would have preferred to have seen him properly hanged—or electrocuted, I suppose, over there… Well, you can't go about having blood feuds and stabbing each other like Corsicans or the Mafia,’ said the Colonel.”

Colonel Arbuthnot offers this opinion about the value of legal punishments during his first interview with Poirot in Part Two, Chapter Eight, and while he seems to advocate for the law over vigilantism in this moment, he also implies that it did not go far enough in Ratchett’s case. He makes a distinction between the chaos of “blood feuds” and the order inherent in a sentence handed down by a judge rather than commenting on the strengths of laws themselves, a position which leaves room for the novel’s final argument about the failures of the legal system.

“‘You are not a believer in Prohibition, Monsieur Hardmann,’ said M. Bouc with a smile. ‘Well,’ said Hardman. ‘I can’t say Prohibition has ever worried me any.’”

As Poirot begins the process of inspecting the passengers’ luggage in Part Two, Chapter Fifteen, he discovers surprising amounts of alcohol stashed among Mr. Hardman’s belongings. Their ensuing discussion about alcohol and prohibition laws in the United States highlights the inability of legislation to completely prevent people from engaging in illegal behavior. As Mr. Hardman’s blasé attitude toward his possession of alcohol suggests, laws have no power if the perpetrator knows how to avoid getting caught.

"It is a conspiracy. You are going to frame me? All for a pig of a man who should have gone to the chair! It was an infamy that he did not. If it had been me—if I had been arrested—"

When Poirot accuses Antonio of lying to him about his true identity as the chauffeur for the Armstrong family in Part Three, Chapter Eight, the Italian man gets flustered and calls out the hypocrisy of Ratchett’s ability to evade punishment for a crime he clearly committed. The point that Antonio begins to make before Poirot cuts him off, that he would have been condemned to death had he been arrested for Daisy’s murder, emphasizes the idea that the law is not enforced equally.

"Ratchett had escaped justice in America. There was no question as to his guilt. I visualized a self-appointed jury of twelve people who condemned him to death and were forced by exigencies of the case to be their own executioners." 

In Part Three, Chapter Nine, Poirot offers his solution to the mystery and explains why he believes this group of twelve passengers gathered to collectively murder Ratchett. This quote, as well as the circumstances of their vigilante act of justice as a whole, highlights the ineffectiveness of laws designed to keep individuals safe. The threat of Ratchett’s predatory violence remained until the twelve passengers came together to stop him, an act which the legal system was incapable of doing.