When it was revealed that Disney expected Kylo Ren, not Rey, to be the breakout star of The Force Awakens, everyone laughed. Of course everyone would fall in love with the villain who's responsible for the death of a fan favorite, and not the protagonist of the movie. Obviously.
But one group was drawn to the series' new villain -- shippers. Although Finn/Poe is definitely the most popular pairing from The Force Awakens, Kylo Ren and Rey are the close second.
Pairing the female lead with a young male villain isn't exactly new. Hermione and Draco were always popular during the height of Harry Potter fandom. But there are troubling implications when a female character is paired not just with an enemy character, but with one who is, for lack of a better phrase, truly on the dark side.
Sometimes, these stories are explorations of the female character "turning dark." But more often than not, they're stories where the Evil Male Character is redeemed by falling in love. Without careful balance, these stories can easily evolve into "female character's love and patience and self-sacrifice brought out the good in the villain and redeemed him," which hovers dangerously close to the message that if the male character then doesn't get redeemed, it's because the female character didn't give him a chance or love him enough.
If that last one seems like a huge leap, people have literally made that argument in real life. When Elliot Rodger went on a shooting spree in Southern California in 2014, he claimed it was to punish women for rejecting him, and commenters then suggested that his actions were therefore those picky women's fault for refusing to date a literal mass murderer.
Of course, people can ship whatever they like. In the end, it isn't real. But when people gravitate toward certain relationships, and when those relationships have such concerning connotations, I think it's important to look at why.
One reason, perhaps, is that mainstream fiction seriously romanticizes these relationships. I've talked extensively before about the relationship between Belle and Rumplestiltskin in Once Upon A Time, where her love is supposed to redeem him, and their story consists of her Believing in the Good in Him, and being manipulated, lied to and generally mistreated as a result. But she feels True Love for him, so she forgives him, allowing him to potentially find redemption that way.
Or in the popular novel, The Wrath and the Dawn, where the protagonist can stop a string of murders just by challenging the murderer, and where his actions get twisted until she is the one in the wrong for even asking more about those deaths, as though it's violating his privacy to inquire about the murders he committed a couple of weeks ago. Again, she has to accept the villain as he is, and that patience and acceptance will lead not only to his salvation, but also to her true love Happily Ever After (rather than, you know, misery and abuse).
And this romanticization of the male villain who just needs someone to love him leads to situations like in Harry Potter, where people criticize Lily Potter for choosing James over Snape, and where we are meant to be believe that his love for her turned him to the good side (but left him willing to see her husband and child die and to bully children for fun).
The worst part of this trope, perhaps, is not just that a female character becomes responsible for a male character's actions, but that she is subsumed by him as well. She becomes an accessory in his story, rather than a player in her own. If she refuses to give herself up to prop up his tale of redemption, she's selfish, a bitch. She has to be willing to sacrifice herself completely in the name of love in order to give the villain reason to do good.
To be clear, this is a very different narrative from one where a female character falls to the dark side, or otherwise explores the ways that she might relate to an evil character. If she toys with the possibility of darkness, she has her own plotline and agency, rather than being a tool for the male character's salvation. But if she's committed to being good and the villain falls for her, she ends up in a trap where she's awful if she rejects him, must put herself at great risk, ignore common sense, and is still seen as culpable for his deeds if everything goes wrong.
So if people want to explore darkside!Rey, more power to them. But stories where Rey saves Kylo Ren from the dark side by seeing his true, sensitive self are quite different. Rey is a fantastic protagonist with a well-developed story of her own, and it's depressing that young fans can see that and subconsciously think, "she'd make a great prop in Kylo Ren's redemption arc."
And even if this is fiction, it has serious consequences in real life. It creates this expectation that girls should always forgive guys and try to see the good in them, at the cost of their own happiness and safety. It suggests that girls have to be self-sacrificing to avoid being selfish, no matter the risk. And it suggests that if everything goes wrong, it's all their own fault for not being good enough.