Kuro no Maou - Chapter 479

Untolerated by heaven, a boy was restricted by the most powerful spell.

He accidentally acquired a painting inhabited by nine souls that claimed to be immortal.

Then, the legend of a strong young man who violated heavenly principles against the course of nature began.

- Description from Novelupdates

Mercy Morgan was just an ordinary young man trying to make sense of his life, but fate had other plans. One fateful night while on the verge of death, he crossed paths with Michelle Elderblood, an ancient vampire whose bite changed everything. Now transformed into Merciless Minerva Elderblood II, he finds himself thrust into a hidden world of powerful vampires and dark secrets.In this realm, where myth collides with reality, Merciless quickly realizes he is caught in a dangerous web of many powerful creatures. Meanwhile, the vampires operate under a singular, formidable king, and every encounter reveals the treacherous politics and hidden agendas that dictate their lives. Yet, amid the chaos, a singular purpose ignites within him: revenge.Merciless barely a day old must face a harrowing exam that plunges him into the depths of forbidden knowledge, forcing him to confront not only the secrets of his new existence but also the gods who have forsaken him. The pain of his past fuels his determination, and as he navigates this perilous path, he wrestles with the question of whether he is merely a pawn in someone else's game before becoming a vampire, or if he is destined for something far greater.Driven by a relentless thirst for vengeance, Merciless must gather strength and confront the darkest parts of his new identity. With every step he takes, the stakes rise, leading him closer to a confrontation that could alter the very fabric of his world.Join Merciless on a gripping journey of discovery, power, and the quest for retribution. In a dark world filled with myth, danger, and relentless ambition, can one man's desire for revenge reshape his destiny, or will it consume him entirely?

The first time they met they had battled fiercely, just for a mere bunch of herbs. She needed it to save people, and he wanted to use it to save his own life.

“Let go of it, or you will die.” The man’s violet eyes were cold and sinister as he said in a low voice.

She held on tightly to the base of the herb and pulled its roots right out of the ground, before she turned her eyes back with a crafty smile. “It’s mine now. Snatch it from me if you can.”

When they met several years later they were no longer on opposing sides in hostility. In order to repay the debt for having stolen the herb back then, she went on to miraculously cure his body’s condition, relieving him of the inhumane torment that wrecked at his body day and night. He was amazed by the godly skills in Medicine she possessed and his heart went on to fall irrecoverably head over heels in love.

- Description from novelupdates

The world’s worst offense, is called Love.