That might be, but still he never expected the relationship between him and ML to even last as long as it did. As what he previously said. And it doesn't matter if they weren't going out for long or not...the fact is he had strong feelings for the hyung. He may be blinded by those feelings. But the fact remains they are there. He needs to make a decision yes but that decision can lead him to not being with either of them. Because if the feelings for the ML was stronger he would have been able to dub hyung and be faithful. But it's not. Honestly speaking, the ML deserves better, Someone that will always chose him.
I can't really blame MC for what he's doing (even though he shouldn't be cheating and should make a decision already) why, because we all have that one ex. If things were different then we would still be together, remember they didnt "breakup". The hyung just moved away. So you cant blame him for still having lingering feelings and that feeling of "what if...". Yes, I do feel bad for ML but that's just a part of life. Relationships dont always work out the way we want. MC could even grow to resent ML in the end. You just have to follow your true feelings in order to be happy. We only have this one life....just be happy, no regrets.
ヾ(❀╹◡╹)ノ~