beta
I think theres 2 outcones to Mincheol's change:
Because Orange Head was bitter that he was consistently a good person and nothing turned it right he tried to make Haesoo the same. Probably inherited the debt, gave up on his dreams, everyone he thought he helped and was friend's with turned their backs on him, and felt neglected by Haesoo. Turned to whatever bad habits that helped him feel better, more alive, and justified it to himself that Haesoo didn't care; that she was just like the rest.
Or
All along he was doing his best to build a big facade because he lived off the popularity, the reputation, and Haesoo helped add to it. She was something he deserved and helped build him up and since we never saw the full expression on his face as he "blushed". Man's was probably getting off on his conquest. He looks awesome fighting a bunch of guys, and saved her from bullies because she would be damaged goods. Setting himself up for a big grandiose life that never came into fruition and just like in the other theory his life fell apart. Since Haesoo was a conquest and wrapped around his finger he stopped caring and put no effort because he grew complacent.
Either way I only see 2 different outcomes and based off what I've seen happen in other stories it's probably the second option.