Because seme has high ego. His lowkey insecurity can't make him to bend down and play the good guy even that just for an act. He need to make uke stay by his side like a servant to serve the master, not an equal lovers relationship. Even when uke left him, he cannot just show the vulnerability as he told uke in a moment before they back together,
"I'll say this just once, and never again. I love you"
When I read it, I was like, why the hell seme must be stubborn with that ugly side of his personality.
That totally aligns with his character in the webtoon thus far. It's crazy growing up and realizing all those bad boys were just insecure little boys
He's willing to do all this plotting, planning, and busting his back but he couldn't be nice and win his loyalty. Which would have been easier since MC's a softie when in debt or someone is in his care. He really could have taken advantage of him if he'd posed as a defenseless child at the beginning or showed dependency. Made him feel that staying with him was actually best for his friends/family. That way the MC would have made excuses for him no matter what he did. But noooooo, he wants to be a bad boy