The Strongest Demon Becomes the Protagonist's Strategist

An overworked game designer dies from exhaustion, and reincarnates into his own game as a Demon Lord who killed the female MC’s family?! Just when he is about to achieve his dream life, the defeat of female MC returns him to the last checkpoint of the game! To retire and live in luxury, he sets on the journey as the main antagonist… to help the MC?! --- The game planner Zou Yu died suddenly during an overtime work and traveled to the immortality game designed by himself, becoming the demon guard who killed the heroine Jiang Yunbo's family. Zou Yu, who thought he would reach the pinnacle of his life in one step, was forced to return to the starting point of the game countless times because of Jiang Yunbo's life-threatening character. It turns out that although he is valuable as a demon, he is still an NPC in the game. This damn Jiang Yunbo is the protagonist… So, Zou Yu embarked on a peculiar adventure where the villain BOSS helps the protagonist grow up (by the way)…