Mufeng decided to enter the Well of Gods and Demons. If he wanted to complete this quest, Chonglou was an existence he could never avoid.
If Longkui and Hongkui wanted to shed their ghostly state, they needed Chonglou's help to gain physical bodies. Even if Chonglou didn't come looking for Mufeng, Mufeng would sooner or later have to go looking for Chonglou.
Since they were bound to cross paths sooner or later, it was better to get it over with early. By now, he was the sect leader of Shushan, representing a faction in the Human Realm. Surely Chonglou wouldn't resort to slaughter the moment he laid eyes on him?
Chapter 300