Naurrrr, this isn't to justify anything. It only explains what decisions he made that led to their suffering before he made the current decisions he did.
It doesn't justify ANYTHING he's done or is currently doing, he's most definitely the bad guy here, but it's not hard to sympathize with him.
Mincheol didn't become a monster overnight, he wasn't born to do what he's doing. He was a lost kid who kept digging himself a deeper hole, and instead of fixing himself he blamed everyone else around him. It's uncomfortable, but he's the most realistic figure in this story for this exact reason.
He's not gaining any redemption or forgiveness for this, it just shows even the most innocent intentions can go wrong, and there's always a wrong path to take afterwards.
Are they trying to justify his cheating or smt if so it doesn't justify any shit he did