I can see where that comes from but the way I saw it was, as he got closer and closer to his "happily ever after" he mellowed out. There was still points made in the later chapters to show his temper, he was just more easy going.
I think the problem was that after the first half (I'd say the car scene with Cain) they basically only really showed him in sex scenes for quite awhile. It probably would have felt less out of character at the end if that part of his transition was explored more. Even just a couple of scenes of the 2 interacting in more normal ways. The story was very focused on other plot developments at the time.
Yeah, I don't fault them too much though because the development in the first half was soooo good. I mean, even with all the sex, we still got closer to knowing him than the MCs in at least half the BL out there. And all the dramatics and passion made it that much more emotional at the end. Really it would be perfect if it had another 50 chapters in the middle ╮( ̄▽ ̄)╭

Am I the only one who kinda dislikes how Do Gyeong Joon‘s personality got reduced to the ‘softie’ type towards the end? I feel like he was more temperamental, maybe it was due to the grief he was holding but now he’s just… flat