He'd still know. He'd take you out to a fancy dinner and regale you with stories of his past accomplishments interspersed with sincere compliments about your attire and the way you wear your hair. Then, the moment the waiter collects the desert plates, he'll say he say just "one more little story" and then lay out your entire murder plan in front of you from start to finish. Everything's there. Even the drunk ex-actor you hired to pretend to be you while you were committing the crime. That guy isn't even here. He was going to show up tomorrow. You haven't even telegrammed to cancel yet. The only thing he couldn't figure out is how you were going to dispose of the clown costume before Lady Ellison found it inside her hat box. Stunned, you tell him you were just going to throw it in the lake weighed down by some rocks.
"Ah, well," he says, a brief look of disappointment crossing his face. "That would have made this case entirely too easy to solve. It is perhaps for the best you have reconsidered, no?"
You ask if he's going to turn you in. He looks genuinely shocked.
"Turn you in for a crime you didn't even commit? Mon Dieu! Not even Hercule Poirot could be successfully doing this!"
"Truly?" you say, your hands shaking slightly. "But...you know I fully intended to kill him. If I hadn't seen you on the train, I would have gone through with it."
"But you did not," he says precisely. "And you will not ever, now that you know the eye of Hercule Poirot will forever be upon you. There is no crime in anger. There is no crime in hatred. There is even no crime in wishing to do the murder. The only crime is in the doing. And Hercule Poirot only concerns himself with crimes."
He looks you in the eye, then. And now you can see it, the truth of this silly little man. Past the little body and the prissy outfits, past the ostentatious moustaches and egg shaped bald pate. You thought yourself so smart, so clever, so righteous in your anger. And he saw through you in an instant. This silly little man who wears his own vainglory like a polished, perfectly straight tie pin would have sent you to the gallows without a second thought.
He didn't need one. He'd figured you out with the first.
"You have chosen not to do. You have done so for the best of reasons, because you recognize the folly of trying to outsmart Hercule Poirot. But even if your reason was not so great, I would celebrate your choice all the same. There is no such thing as the bad reason to choose against staining your soul with murder."
With a polite smile, he pays for your meal and leaves before you can say anything else. You never see him again. The next morning you book passage back home. You see the man you were going to kill occasionally, out and about. But for some reason the old anger you always felt in his presence, the righteous fury that burned in your breast at the very whiff of his cologne, the pain that tore through your body every time you thought of how he has wronged you, never returns.
He is just a person, you realize. A loathsome person, but a person nevertheless. In the end, it is not your place to judge him. It had never been your place. His judgement rests in the hands of someone much wiser and more just than you could ever be. Than even the great Hercule Poirot could hope to be.
You never see that silly little man again. But you keep him in your prayers for the rest of your life.
He kept you from making the worst mistake you ever could have made. And you are grateful.