Ok so this kid is like "wow that's bs" because I guess the died early or something so they didn't have the time in life to prove they were worthy of being a demon... BUT some old demon is like "yeah, but because you died and didn't get to live your entire life, you have extra time before passing to prove yourself!"

This kid is like "yeah I'll do that, but I think that's kinda dumb anyway"
So he goes to destroy the devil/god/gods/whatever-being(s)-is/are-in-charge
because the idea of having to prove yourself is stupid to him.

Cool thing is, if you destroy the devil you basically get to take their place, so boom, he's off to destroy them in order to create the new law of the underworld, where nobody has to pass from the underworld.

This is like the opposite of Kid Icarus.

Kid Hades?

d/c again:
Ok so basically... (sorry if I offend anyone) I like to make religions or use existing religions in a lot of my stories, but they usually involve destroying the higher power because of corruption disguised as "justice" based on classifying humans into right and wrong and because the diety of such religions don't allow true and honest redemption. Also they often have arbitrary classifications of "right and wrong" and "good and bad"...

Any beings of any existing religion/belief system can count as the ruler of the underworld in this case, since I guess I like to imagine (in the sense of this being a game) the "final boss" is represented as simply a concept, so it takes the form of any diety it is related to.