Zui Qiang Mozun De Tuixiu Shenghuo Cong Gonglüe Zhujiao Kaishi

Game designer Zhou Yuzai suddenly died while working overtime and transmigrated into the cultivation game he was working on. He became the main antagonist Wei Buwang, who had killed the female lead Jiang Yunbo's entire family. Zhou Yuzai thought he would have it easy in his own game, but Jiang Yunbo's tendency to die in game also reset him back to the start, too. It turned out that even though he was a powerful demon lord, he was still just an NPC compared to the actual main character Jiang Yunbo. So he decided to help Jiang Yunbo get stronger instead, and pick up his own harem along the way!
(Source: Bilibili, translated)
Notes: Includes one bonus chapter.