Sekai Saikou no Ansatsusha, Isekai Kizoku ni Tensei suru

The most lethal assassin in history had vowed eternal allegiance to the group that had reared him. But in spite of his loyalty, that very group acts to suppress him, which finally results in his death. He finds himself in front of a goddess who has been drawn to him by his amazing abilities while drowning in frustration and regrets he can no longer repress. The goddess offers him the chance to reincarnate into a glorious world of magic and swords in order to complete a vital task: kill the world's hero in order to stop it from being destroyed. He relives as Lugh Tuatha Dé, the son of a noble line of assassins serving the Alvan Kingdom, after agreeing to the goddess' request. Lugh is taught new murder methods by his father, which are very different from the cold-blooded and callous killing methods of his past existence. His other skills also develop, which enables him to make new friends and allies. Even so, Lugh is aware that his efforts fall well short of what is required to completely vanquish such a formidable foe as the hero.