In both fiction and roleplaying games, there's a distinction between a character (or NPC) who is a villain, and one who is an antagonist.

The villain is evil, or at least selfish and callous. Not only do his (or her) goals oppose those of the heroes, his character is morally inferior. Even if he believes he is serving some lofty goal, he's willing to let the ends justify the means. A villain may be monstrous, or charming, or sinister, or deceptive, but he's the "bad guy."

The antagonist isn't evil, at least not consciously. Lieutenant Gerard in The Fugitive is a perfect example. He's hunting a dangerous escaped murderer, and is determined to bring him in. It's a noble and worthy goal, and he displays courage, competence, and integrity as he pursues our wrongly-convicted hero.  

Naturally, an antagonist is the most likely to reverse himself upon learning the truth. If he doesn't, he slides over into being a cruel fanatic like Inspector Javert from Les Miserables, who is a villain. Or, if it's a tragedy, the antagonist may learn his error and repent too late.

Antagonists are often praised as being "realistic," since we're constantly assured that in the real world everyone considers himself a good person. Whereas villains are disparaged as being unrealistic comic-book characters.

That's false, by the way. There are indeed people in the world who are consciously wicked. I recall running across an interview with a mid-level Mafia "soldier" in which he mentioned how much he liked to go out on spending sprees with his girlfriend using a stolen credit card. The Mafioso was making thousands off of his various rackets already, but the knowledge that some "sucker" was paying the bill for his entertainment made it that much more enjoyable.

It's also a fact is that many people will do wrong if they think they can get away with it. Think about most theft, or robbery. Think about actual premeditated murder. There's no doubt in the robber's mind that robbery is wrong. Killers know killing is wrong. They just don't expect to get caught.

And of course there are people who are so callous or incapable of empathy that it simply doesn't matter to them if their actions are harmful. Actual psychopaths have that problem — and they may make up as much as one percent of the population.

I'm not trying to paint a grim picture of humanity. It's noteworthy that crime has been declining continuously during most of my lifetime. (I'm not saying there's a connection, but you're welcome.) Some of that is due to better deterrence — fewer people think they can get away with wrongdoing, so they don't do it. And maybe some of it is due to improvements in empathy and moral sense. I don't know if there's been any research on that. (One colleague of mine attributes the decline in lawbreaking to the increase in obesity over the same period: would-be crooks are just getting too fat to commit crimes.)

What this does mean is that writers shouldn't be ashamed to have real villains in their stories. It's still a good idea to make the villains three-dimensional, with realistic desires and maybe some redeeming qualities. But in the end, good and evil are real, and one reason people have been telling stories for half a million years is to remind ourselves that good can win.