Honestly I had a hunch that hakyoung acted the way he is because he was raised with the "you'll be the heir! You'll inherit this legacy!" attitude from the mama. Turns out it seems like he indeed was raised that way, with how mama acts and behave especially the stark contrast between the way she behaves with the two brothers. So Hakyoung grew up believing he's the sole heir— no rivalry whatsoever. That inheritance was his and solely his.
..that was until Yigon was born. That means Hakyoung has to pretend to be a good big bro to impress the adults. "Ohh, what a good brother. He sets a good example for yigon. Perhaps this business will thrive under his leadership(?)." Then, he has to find some ways to secure that inheritance. From then on it turned into obsession. So far we see some batshit crazy manipulation since childhood, then in current timeline it's bugging yigon's house and super thorough background check & stalking. Oh, let's not forget violence too. Mf really paid people to be his punchbags. Then some threats, blackmails(?), and I take the apple part as a murder attempt. Who knows what more he has up in his sleeves? All for the sake of that inheritance. It's as if without it he would be nothing.
But the fact that grandpa favours Yigon to the point of contradicting his own principles could mean two things : 1. He saw Yigon's potential and doesn't want to waste that talent and 2. He knows the mom is probably scheming something (judging from how obsessed she is with wanting hakyoung to be the heir).
AHHH it's so hard to see a red flag like him in manhwa
WAITTT I MEAN GREEN FLAG SHIT