Although Dutian Devil King planned to head towards the Great Void to achieve deeds of merit, he was, in fact, seeking his own death. However, he had already entrusted his people to Qin Mu, and if he died in the Great Void, Qin Mu would have no choice but to take care of them. To him, this was also considered as his wish being fulfilled.
In his heart, he knew that the high and mighty Celestial Venerable Mu would have the means to settle his people in a safe place.
Qin Mu may not provide the utmost care to his people after his death. However, based on their past relationship, he would at least give them a place to settle down so that they wouldn't be drifting in the universe, putting themselves in unpredictable and dangerous environments.
The heavenly devil people had enough of their years of wandering.