>>281455746
It's quite reasonable when you stop to think about it.
Denji wanted to be weaponized because he felt guilty, but now he knows Yoru wants to do it to create Hell on Earth, so stopping her is just common sense.
But ultimately, the only one who can stop Yoru, and save both Denji from being turned into a weapon and the world from being turned into a hellscape, is Asa, who is inside of her.