When the Bad Guys Sell their Cause a Little too Well

    I read a novel a little while ago that I was really disappointed in by the end. The premise was fascinating, and the setting and background story were genius, but I felt really let down by the main protagonist, who goes from being a really nice and noble person at the beginning to a coldhearted, corrupted murderer in the end. It wasn't even a tragedy story like Marissa Meyer's Fairest, where the character is expected to turn to the dark side over the course of the story; this was very sudden, and frankly out of character.
    One of the major factors that led up to this character's moral downfall (and another thing about the novel that saddened me as a writer and a person) was the way that the antagonists, the cruel and self-centered villains, presented their cause. There were multiple incidents over the course of the story where the main character was hearing the antagonists' view on a conflict both their sides were in. The antagonists' points were realistic, actually very important things to consider ethically, but when the cruel villains finished talking, everything was just left at that. There was no rebuttal from the main character, or from another one of the good guys; nothing happened in the story that provided a ray of hope, or any kind of proof that showed that the evil deeds of the antagonists were unjustified. By the end of the story (and even well before that), the villains' cause sounded righteous, and the virtuous side from whence the main character came sounded like the wrong one.
    Usually it's a pretty good writing accomplishment when a bad guy can sell their cause really well--it is logical for an antagonist to think they're right, after all; and when their corrupted logic can tempt the character, it's interesting for plot. But in this kind of situation where the bad guys had the final word when they didn't have to, and their claims sounded right when there were plenty of arguments that could have shown that they were wrong, I think the author really crossed a line. Because not only was the main character listening to the villains' argument, and watching their crooked claims go without defiance, but so was the reader.
   And while people don't believe everything they hear, stories still have a special sort of power that can potentially sway a person's views. Characters might not be "real" in the sense that they aren't walking on our streets, and the settings might not actually be established places on earth, but the principles of the story's themes, and the way that a reader can connect with characters is very real. So when a person writes a story with a great (at least, initially great) protagonist/narrator, and makes an un-refuted claim that  willfully evil actions can be justified and accepted, or that genuine goodness doesn't even exist, it's possible that a reader will make some partial connections to the real world and really accept this as truth. And that is a seriously horrible and tragic thing to happen.
    There are a lot of books where the established "heroes" of a society really do turn out to be corrupt, and where the villains do a top-rate job at persuading people to their side, but those stories don't push it too far like this book I read did. They don't absolutely blot out the line between good and evil. They show that the world can be a pretty corrupt place, but they also provide that goodness and truth can still prevail. They show how there is a difference between right and wrong. They let their antagonists make seemingly bulletproof arguments, but then they bring in someone who points out all the cracks and flaws and outright lies in it, who gives an argument of their own (or prompts the narrator to create one) that stands for truth, virtue, and the good things that the world really needs to stick to, not abandon.
    There are ways to apply the skill of persuasion to antagonists without going too far, but it is important to make sure that they don't go too far. Does anyone else agree with me on this?
    Have you read any novels where the antagonists' cause stands unquestioned? Or have you read a book that applies a healthy measure of persuasion, where the antagonists sell their cause really well, but not too well?