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?!