
Here it is important to pay attention to the clues about his childhood. A super-inadequate tough father who abused him, an equally inadequate older brother who seems to want him to die. And - the absence of a mother figure. Such guys often fall for older women.
And here, for the obsession to blossom, it didn’t take much - she showed him care and kindness, purely maternal. And for him it was so abnormal, unusual, that the “suspension bridge” effect occurred, or how girls fall in love with the firefighters who saved them.
Puberty added sexual attraction, and we have what we have.
Plus the guy is a little uh ... neurodivergent due to his traumas and terribly romanticizes all this. It is important to him that she chooses him, it is important for him to be loved. And she is exactly the person who knows how to love like that. He wants Minchol's place for a reason.

You have very good points and I totally agree with you, but when did she even show him care and kindness, she didn't even pay attention to his existence at all, she was always obsessed with her husband, and whenever he was literally in front of her she didn't even seem to realize his presence, even tho every girl and old women too noticed him immediately due to his tall and handsome appearance, but she never paid attention at his existence at all. So for me it still doesn't make sense his obsession for her. Unless it's due to the fact that she doesn't react to his presence like all girls do, but still not enough for a reason. Or maybe the way he saw how she loves and cares for her husband so it made him wanna take his place? Still wouldn't make sense bc he would need to stalk her alot to know that about her (so it means he would already be obsessed with her), and also it seems through the story that he tried at some point to give her husband chances to change and become better, to make her happy. So it still makes no sense no matter how I look at it
I don't understand why the ML is so obsessed with the MC?? Did they ever explain it? It seems to me like he pitied her and wanted to protect her ??