I Was Supposed to Be the Villain, So How Did I Become the Protagonist's Lapdog?

[The rating is low because this just came out. It'll go up.] Come watch two yanderes fall in love. A love-averse shou who only gets close to the gong, with an unusual way of thinking, versus a gong who overthinks himself into love, with an even more unusual way of thinking. Ye Yunqing was born with an emotional disorder. After transmigrating into a book as its savior protagonist, he was too cold and out of character, so the System saddled him with a frail-body debuff. A once peerless prodigy became a sickly invalid and switched to studying medicine. When mortals are corrupted by demons, they turn into monsters. Ye Yunqing loves studying monsters. That is, until he met Lin Wang, who could keep his sanity even after being corrupted. A hundred years ago, the two had competed for the title of the Immortal Alliance's top prodigy. They fought countless times without a clear winner, and Lin Wang spent a century in seclusion, waiting for the day he could emerge and fight again. But Ye Yunqing had become a sickly invalid, while Lin Wang had become one of those so-called monsters, and Ye Yunqing even took him home to raise. “This little monster is sane. Let's study why.” “This little monster looks listless. Let's try feeding him some aphrodisiac and see what happens.” “You're the little monster I raised. Be cute. I don't like scary ones.” Ye Yunqing only got close to his little monster, unaware that Lin Wang was the yandere villain in the book who would destroy the world. Day one, Lin Wang: My sworn enemy is humiliating me? I'd rather die than suffer this disgrace! Day two, Lin Wang: He smells so good. I want a taste. Later... Lin Wang: Demons are causing trouble. Give me a kiss and I'll take care of it. Later still... Lin Wang united the demons and became their great overlord. He immediately snatched Ye Yunqing back home. “Be my cultivation partner. You're not allowed to talk to anyone but me!” But Ye Yunqing used a formation to hold him back. “Little monster, you're mine. Don't go snatching anyone else.” Lin Wang, who'd been ready to take him by force: Wait, which one of us is supposed to be crazy?! System: Good grief, a pair of yanderes who are mutually in love! You two sure know how to make romance exciting!