So I'm writing a story where the main character is sent to kill the main villain. I have the basic world set out before me, and now I focus on story. I was planning out the main storyline and I hit a bit of a snag... uh-oh. I was thinking that the main character could get to the "villain" and go through that painful talk that all villains feel like they need to have, you know, where the hero is tied up, and they reveal their plan for some reason. But this time I was thinking it would actually work, and the hero would start working with the main "villain".

I initially thought that I wouldn't be cheated but then I started thinking about how I would actually react to this happening. Like, how upset I would be if I sat there reading this book, growing a hatred for the villain, and then all of a sudden I'm supposed to like them.

This brings me to my question. If I wrote this story this way, with the villain being successful in "the talk", would readers feel cheated out of half of a book or can it be done in a way that makes it a good experience?

It could be interesting. Not all villains seem inclined to the talk, but having the hero listen and realize he was wrong could be refreshing.

Assuming your villain is a three dimensional character and your MC is likewise fully fleshed, there is no reason for this to feel a cheat. It would be a twist and likely change the direction of your story, but that can be a good thing.

Making a change of world view a logical move will be delicate and intriguing. What might induce the hero hell bent on destroying the villian to have an epiphany?

In one work I have set aside for the time, I was well into it when I discovered that my hero was actually an unknowing villain and the person he feared and hated the most was actually more of a reluctant hero. If my original ‘hero’ were not so blinded by his own prejudice, he could be persuaded that he was mistaken regarding the other man.

I mention that because your character must be capable of listening and hearing the truth even when spoken by the alleged villain.

The villain doesn't need to convince the protagonist as much as

you need to convince the reader.

If you can make it believeable to the reader that the protagonist changes sides, then it will be a satisfying read.

We often find out that our suspicions were mistaken in real life. For example, quite often a law enforcement agency finds out that their suspect was innocent. Why shouldn't that happen in a novel?

But you might also show how the villain manipulates the protagonist succesfully into believing an untruth. Protagonists don't always succeed, and this might be a story about a protagonist succumbing to some flaw. This too is something that happens in real life, where we often like to believe what we think should be true, but in fact isn't. If you manage to narrate this process convincingly, the reader will gladly follow you to your unhappy ending.

It's an unhappy ending if good does not triumph over evil.

Books and movies with unhappy endings are generally frowned upon, they tend to do poorly commercially. The successes amongst unhappy endings tend to be highly emotional, understandable cautionary tales; which means in a twisted way good still triumphs over evil, or at least evil leads somebody to a terrible end. The reader is following along with a character they like that descends into drug addiction, or crime, or losing their marriage, or corruption, etc, but they end up miserable or dead in the process.

The Escape Hatch ... Flip The Script

AKA role reversal. The villain proves to the MC that the MC has been duped, that he is a pawn of the people he thinks he is helping, or his management, etc. The villain doesn't convince the MC to be evil, the villain convinces the MC he has been working toward evil ends and the villain is doing good.

And instead of insisting the MC change sides, the villain gives him a chance to verify all this himself. After telling him all this, the villain, with the MC bound and a knife to his neck, says "I could kill you, and eliminate a threat. Remember that. But I won't, because I don't think you are evil. I'm going to set you free. Come back when you're ready to fight for what's right."

The reader following the MC is not disappointed, the twist does not mean the MC is now fighting on the side of evil. It means that the reader, like the MC, was duped into believing the villain was evil, but now understands the villain was good and the real villain is the queen that sent the MC out in the first place, trying to get rid of him so she can kill her husband and take power.

But the villain didn't kill the MC as she expected, the villain just pulled back the curtain to expose the queen as the real villain all along, and now the reader can believe the MC is still fighting for good, and the defeat of the queen represents the triumph of good over evil, so this is a happy ending.

Another Escape Hatch: Double Reversal.

A double reverse means the protagonist succumbs to the temptations of evil, but the "good" inside him eventually reasserts itself, and he reverse again: To the side of good, and defeats the villain after all.

Falling for the villain's talk is just another obstacle and failure along the way, this time within himself. You have a flawed protagonist. The next story is not exactly your situation, but consider a good cop. In a time of incredible financial hardship (e.g. he can't pay for the treatment his wife/mother/kid needs) he becomes a dirty cop. He gets deeper and deeper into being a dirty cop, until he accidentally shoots and kills an innocent witness he was trying to keep from exposing him. He covers that up successfully, but it weighs on him so much, he sacrifices himself in a blaze of glory to become, once again, a good cop.

Evil Triumphs In The End is not generally a story people like. It is depressing, and we read fiction to escape the real world, where evil often does triumph in the end, criminals and murderers and rapists and frauds get away with their predations, get insanely wealthy and/or powerful, and are never punished, dying peacefully in their sleep without a regret in the world.

You can write it if you want, perhaps it will be some kind of catharsis, but if your goal is to entertain people and have them like your story, then I'd suggest evil can be wildly successful in your story, but in the end, good must triumph over evil.