We are at one then, said Poirot, "for I, too, want to hang the criminal."
Poirot says he wants to hang the criminal, too, though only after a fair and complete trial. 
mnli hypothesis: Poirot says he wants to hang the criminal, too, though only after a fair and complete trial.  premise: We are at one then, said Poirot, "for I, too, want to hang the criminal."