Hercule Poirot was a Belgian detective.
After solving the murder, Poirot damned the criminals for taking the law into their own hands. Mary Debenham attempted to make him see that the crime was justified by Cassetti's evil doings. When the police arrived he lied, telling them that the murderer had escaped and was still on the loose and allowing the criminals to go free. As he walked away he still worried over his choice to not turn them in. (S12: Murder on the Orient Express)