I thought that he would try to free them from Makima's control then make a weapon-human hybrid superhero team and then we would have our happy ending....... I was wrong he just chainsawed them for the second time.
But I have a theory! Remember when Makima said that " there was probably a way to save Aki" ? What if she wasn't bluffing.
Looking at Aki's situation we know that devilmen/ feind have the original person still inside, specifically their deepest desire, and their actions in their "heaven " can impact the behavior of their body even when it's taken over
In chapter 39 Makima (The control feind) only cried at the movie that showed a reunion between brothers or clode friends. What if the person whose body the control devil possessed lacked a close bond with a person in their life and that caused Makima to cry?
Denji could be that person for Makima and make a contract in which he will , but as part of it this can be the only contract that Makima can have.
And then we get a sequel!