We need to know why the heck did mincheol change he was such a green flag what happened

It's because he's chosen a temporary pleasure over long time relationship （￣へ￣）there is no further answer to that. He might have been a green flag before but people cheats if they want to. He cheated because he's found a easier lady to hang on

For a man to change like that after marriage is actually weirdly realistic, happens all the time. They turn their own insecurities into resentment and blame their wives, they think being awful to them will bring back the honeymoon phase or maybe give them power over them. It's usually a power trip