Part 1 was largely about CONTROL with Denji's life being at others' control, part 2 is all about WAR
Denji's war within himself about living a normal life vs being CSM
Asa's war within herself (literally & figuratively) about doing the right thing vs living more selfishly symbolically represented by Yoru
Denji being drawn to both the altruistic/down to earth side of Asa and chaotic/sexually assertive side of Yoru. Highlighting Denji's inability to choose between two choices and separate love and abuse
Asayoru being respectively drawn to the caring/sentimental and hedonistic/miserable & affection craving sides of Denji. A perfect way to depict the War Devil's conflictual nature within herself
Yoru being the final antagonist makes a lot of sense from both narrative and thematical standpoints
She's literally the embodiment of the very concept Denji and Asa's struggles tie into and how their war within (plus external factors) prevents the MCs to get close to each other