He didn't simply "become" a terrible person; you could tell early signs that he's always been a reckless, impulsive, & self-centered dude who prioritize his own feelings over FL's will. He quite aggressively pursued her into a relationship (despite her rejections) and helped her out partly out of his hero complex (ie, to make himself feel good). I can't get over that he did not stop or hold back during their first night even when FL was crying & hurting (meanwhile, see how taeha is good at holding back).

People are getting pissed because characters aren't 100% evil and have a reason to have become shit
I don't think there's anything that justifies abuse, but it's interesting seeing how Mincheol became a terrible person
If a story only has one dimensional characters, it's a boring one
Complex characters are great for angsty, bitter storylines