Monogatari no Kuromaku ni Tensei Shite

Ren thought his reward for being the first to clear the sequel to The Legend of the Seven Heroes would be personal satisfaction. So when he's rewarded with “special content,” he quickly accepts...only to wake up as a baby in the game's world. At first, hoping he's been reborn as the hero, Ren instead discovers his new life is that of the story's shadowy antagonist, the figure suspected to be the mastermind behind the entire trilogy. 

Good thing Ren is fully intent on living a peaceful life in the countryside—without getting involved with any characters or world-ending plots. But when he crosses paths with the very saint he is meant to kill in canon, Ren realizes fate may be the ultimate final boss. 

(Source: J-Novel Club)