Just The Way He Shows That He Loves Her
He may stalk her, stated to be either out of worry for her safety or because he can't stand being away from her. He gets violent with romantic rivals, or sometimes even a guy who she merely interacts with. He gets even more violent, murderously so, with anyone who hurts her. He's extremely controlling towards her.
And this kind of guy is seen as romantic. An ideal partner. It doesn't matter if he almost kills her nice male friend, or if she has no privacy, or she feels like she'd 'dating the Godfather'. He's doing it because he loves her, so it must be OK.
There's a stereotype that abusive spouses are just plain bad people. That they don't care about their spouse's feelings, that they fake remorse to get her to stay. And that all abusers must be physically violent towards their spouse - the threat of violence, or emotional manipulation don't count as abuse.
In reality, many abusers have borderline personality disorder*. People with borderline personality disorder can love others, in fact their love is generally too intense rather than the opposite. They feel everything way too intensely, and can't control their reactions. And one of their biggest fears is being abandoned.
Abuse typically involves a build-up, the abuse, and then a 'honeymoon period' where the abuser is apologetic and loving. Typical portrayals have suggested that the honeymoon period is manipulative. That's true for some abusers, but it's not true for borderline abusers. When they apologize and try to make up for what they did, they really are sorry about it. But they have no idea how to stop themselves from doing it again.
The idea that all abusers are cold, unfeeling monsters is a dangerous one. Because when it's obvious that he really does love her, that he's not a cold, unfeeling monster, then people tend to assume he can't be an abuser. No matter what he does. Especially if he never actually hits her, if his abuse consists only of threats and emotional abuse.
One big reason why people perpetuate this myth is to convince abused women to leave the relationship. She often stays because of the love, and because she pities him and wants to help him. If you can convince her that he doesn't love her and she can't help him, presumably, she'll leave.
But the thing is, a person doesn't need to be cold and uncaring to be a danger to you, and someone you're not able to help by staying with. Borderline personality is a hard condition to treat, and a hard condition to live with. And the person must realize something is wrong with them and want to change in order to make much progress in actually changing. Many borderlines think their reactions are just to be expected, especially since borderline personality disorder is typically caused by abuse.
And fiction writers should stop buying into the myth that abusive behavior is romantic, and the kind of thing an ideal lover does.
* Note: This doesn't mean all people with borderline personality are abusers. The traits of this condition make abusive behavior more likely, but not guaranteed. All borderlines are unique individuals, and they'll express their problems differently.