Children of the Wasteland by Lucifier

Hugh Aaron, a murderous teenage prodigy, wakes in a padded cell to learn he is the last survivor of a holy apocalypse. Armed only with his late boyfriend’s spell book, Hugh battles angelic abominations on a desperate quest to usurp the God of Creation and resurrect his lost love. To aid in this conflict, Lucifer, the enigmatic King of Hell, presents a wager: If Hugh can overcome his mortal weaknesses in an eldritch death game, Hell’s demonic forces will serve as his devout soldiers. The targets he must murder hold his deepest emotional vulnerabilities and the secret to divine ascension. Long past pesky moral objections, Hugh is more than up to the task if it means achieving the happy life he was denied. The question is: Can he become the perfect monstrous goddess he always envisioned himself to be or will his fragile heart swallow him before the wasteland does?

"You want transgression? Here's your transgression. Bolder than all the babyblendering dudebros by half and more creative by a wholeass sea of brimstone truth"
- Garrett Cook, author of Charcoal and Kennel