His parents kinda left him empty and broken at such a young age. A tragedy it had to happen to sa-chan among all the people in the world. Also what kuroi said was right, he is worse since he /is/ cheating. He exactly grew up like his father he disliked.
This kind of angst, regret after damage isn't new but I'm looking forward how it play out knowing the spoilers I've seen. This might even go up to 40-50 chapters. Looking forward. (〜￣△￣)〜
It's only up to few chapters! The rest is paid;
https:// manga . bilibili . com /m/detail/mc28029