question about the murder on the Orient Express

kind people , does anyone know any books like the ending (SPOILER) of the murder in the Orient express where Poirot let's go of the murderers just because they were morally just? Or was this the first book in its time to deviate from the said punishment of the murderers . Ps. I have to do an assignment paper on it and want to know how to approach or critique or what point to make so if u also know any interesting facts even about Agathe Christie if would be of huge huge help to me . Extra scared cuz the professor kinda hate me so any help would be appreciated kind people