Curiosity Causes Conversion

There are few things as deadly and unpredictable as curiosity. However, what's most dangerous about curiosity is its ability to impart a driving desire to know into a character. So when an evil character who cannot comprehend good is on the receiving end of kindness, love, or mercy, she will get an insistent and annoying urge to know just why the hero and his cadre have done such an inexplicable thing. There's no way this Worthy Opponent would do something as stupid as save or help an enemy, she'll reason. She'll assume he's being The Chessmaster and is trying to manipulate her, perhaps foolishly expecting her to feel she "owes him one."

So it eats away at her. She'll "study" good to try and understand how the hero will turn such an unfathomably idiotic act to his benefit. As the obsession grows, she'll probably spy on the hero and slowly develop a bit of a crush. She'll try getting close to him and understanding his actions by donning a disguise and doing minor good things, only to discover to her horror that Good Feels Good and she's Becoming the Mask, until eventually she does a Heel Face Turn.

In a weird, unintentional and roundabout way, by helping her the hero has managed to manipulate her to his advantage. And they say good is dumb.

This trope can exist between cultures as well as people of different morality. When people from two different cultures meet, a character from one culture may grow fascinated with the other over a small trinket, casual observation, or snippet of info. As he learns more the fascination grows, giving him a frame of reference with which to compare his own culture. This usually results in him realizing he's unhappy in his own culture, and leads to him defecting. Don't expect everyone on either side to be too thrilled about this.

Anime and Manga

 * In Macross, this along with Humanity Is Infectious caused three Zentradi spies to go native.
 * Also a somewhat similar thing with The Ace on the enemy's side who tries to study and defeat The Ace on the human side.

Comics
"Pagan Man: Man, how do they do that? Pagan Woman: Must...find...out..."
 * In Cartoon History of the Universe, a pagan Roman couple in the arena watch with fascination as a Christian martyr enthusiastically welcomes death by lion ("C'mon! What are you waiting for?"):


 * This happens all the time in Jack Chick tracts. Ala "Who is Jesus? Tell me more!" and a few panels later they're crying and begging for forgiveness.

Film

 * John Preston in Equilibrium is an interesting case. He was a loyal member of the Tetragrammaton, but he understands how people who aren't on emotion suppressants act, sort of intuiting emotion. After meeting a resistance member who reminded him of his dead wife, and killing his partner for going off the emotion inhibiting drugs, he accidentally-ish broke the morning vial emotion suppressants. Hello emotion!
 * In The Nightmare Before Christmas, Jack Skellington is deathly bored with running Halloween, so when he accidentally winds up in Christmasland he becomes morbidly curious as to how Christmas "works". Facing Blue and Orange Morality, he conducts experiments trying to measure the marigolds to quantify merriment, deduce generosity, and distill happiness. Eventually he embraces his image of the concept so thoroughly he tries to save Santa the hassle of running Christmas. It went Horribly Wrong.
 * In other words, this trope is deconstructed. Also, one should note that unlike most trope examples, Jack isn't evil.

Live Action TV

 * In Power Rangers Mystic Force, Itassis wonders how the Power Rangers keep beating the Ten Terrors, as the Terrors are more powerful than the Rangers are. Being more of a Anti-Villain than the others, she goes to the Rangers and asks them their secret. They answer "courage", and she eventually abandons the Terrors to learn more.

Tabletop Games

 * This is a central mechanic for the True Fae in Changeling: The Lost. As primordial chaotic beings, they can't understand human things like altruism or love; they just view them as passing fancies to be tossed aside when convenient rather than defining forces. In trying to understand these bedrocks of humanity, they become human, cutting themselves off from their fae memories and a good chunk of the powers. Of course, in most cases, that only lasts as long as they don't start getting curious about fae existence...

Western Animation

 * Teen Titans takes this for a much quicker spin than most. During Starfire's introduction, Robin is nice to her, and she is (understandably?) confused. He explains, and she claims the closest word her people have is "weak". Cue the middle of the episode, and she's seen the light.

Real Life

 * Many colonization efforts in early America went this route-
 * 17th century England was a generally lousy place to live, with ridiculous social stratification, an incredibly corrupt legal system, and extremely unsanitary living conditions. As a result, a major problem with early American settlements was that colonists (many of whom had come from debtor prisons) were working for corrupt, incompetent officials who had no idea how to run a colony. Meanwhile, the Indians who lived a few miles away were doing just fine even though they didn't have the "advantage" of brutal flogging for minor offenses. Those who learned how the Indians did it quickly realized that it was a vast improvement over their current way of life and promptly left.
 * One particular element of this is that Native American cultures were far more egalitarian than their European equivalents, so what few women came to the New World were much more likely to ditch than the men were. It was this that ultimately caused arriving European settlers to lose their curiosity, as colonial authorities quickly concluded that the women were being "kidnapped" and made sure that any they "rescued" stuck to this narrative. Sadly, it was centuries before this interpretation of events was given any serious scrutiny, and guess who suffered in the meantime?
 * Even after this, African slaves still made a run for Indian communities, for the obvious reason that being a slave sucked and they weren't all that quick to believe that Massa was telling the truth about Indians being savages, since Massa himself was a complete asshole. This is most famously the origin of the Black Seminoles. It should be noted that one of the big advantages in expanding the American frontier is that the bigger America got, the harder it was for slaves to run away to Indian tribes and become nearly unfindable.
 * The fact that their captors would've also referred to the slaves' own freeborn African ancestors as "savages" probably also gave would-be runaways an entirely-different impression of what that word connoted.
 * This also explains how so many current African Americans can find Indian ancestry in their lineage.
 * In fact, one might be surprised upon meeting a man as black as the ace of spades to learn that he identifies himself as Cherokee.
 * This can also happen regarding religion, but not quite as easily as is sometimes portrayed.