>>280930229
Death Devil represents the end point for Denji's current hedonistic lifestyle. A soulless husk who only lives for eating and fucking around.
Nothing fulfills her, and since she's immortal, she cannot even die.
Denji already wants to die at 18 years of age or whatever, now imagine being Death who has lived presumably for hundreds of thousands of years.
Her motives for saving humanity are as simple as Denji's motives for being a hero, she just wants to keep indulging herself at humanity's expense.

>>280822281
That's a tough nut to crack