In the real world, although we humans can have strong and passionate disagreements, I can’t imagine that anyone really thinks of themselves as the villain, purposefully setting out to get in some hero’s way. Rather, villains are just doing what they honestly think is right. If their conscience annoys them, they don’t think about it, or rationalize it away.
This is why villain monologues also seem unrealistic to me. They come from thinking of a story only through the hero’s point of view. “The poor hero is being persecuted! It’s all the villain’s fault! He’s just so hateful!” What about the villain’s point of view? Can’t he be just as passionate about something as the hero? I don’t mean a story should be morally ambiguous. I mean that a villain should be just as human as the hero, and his motivations should be sympathetic, even if we as an audience disagree with his ultimate choices. That is, even if we disagree with his decisions, we should understand where he’s coming from. In this way, a monologue at a story’s climax should not be necessary; his motivations should be apparent from the story.
But there is a time when I think something like a monologue can work. A monologue is meant for exposition, so that we as an audience can understand what and why a villain did what he did. And while a monologue itself is unrealistic, it is conceivable that a hero may, during a conversation, coax a villain into revealing his motivations. This can only happen believably if the villain thinks the hero may be sympathetic to his views. For example, the villain may be trying to lure the hero into joining him. Or perhaps he just wants sympathy and validation. (I recall Stinky Pete’s exposition in Toy Story 2, or Darth Vader’s classic confession in Star Wars: The Empire Strikes Back.) Such a conversation, when the villain is trying to get something out of his exposition, should seem more natural than a direct gloating monologue spoken only for the sake of exposition.