Nocturne's plot is about the world having ended and you're to help create a new one, picking one of the people with a concept for a new society to side with or alternatively bringing back the old world or leaving the remains of the world to stagnate forever. Almost every NPC expects you to be doing this, with the exception of Lucifer, who you have no reason to trust: he stuck a parasite in your eye to turn you into a demon because "your suffering is [his] delight."
Lucifer asks you to go explore a lengthy optional dungeon filled with grinding and collect all the candelabra to light the way by beating up a lot of difficult optional bosses. There is very little incentive for you to actually do this beyond a bit of worldbuilding and backstory for some of the side characters, and it involves hours of grinding and exploring this labyrinth as Metatron and many other characters tell you to leave this place.
You are specifically told by everyone even remotely reputable that you want to leave it alone, and also threatened by the four horsemen of the apocalypse should you decide to continue onward past a certain point. Some of the puzzles in there are truly cruelly designed and frankly delightful as a gameplay choice.
If you get down to the bottom, you complete your transition into becoming a demon and gain a demonic soul to go with your demonic body. You went down there because you could even though you really shouldn't, and then after you do this, you kill every character you could have aligned yourself with, and then you kill God's observer in the cycle of rebirth, and then you join Lucifer to destroy all of Creation.
In exchange, you get ... a bit of content, and in my opinion the most emotionally satisfying ending, because the whole game's about cycles and people in destructive patterns who are prevented by circumstance to break out of it. The game as set up is cruel and the people who learned kindness all died as you could do nothing. Why not burn it all down?
Well, Undertale takes this and replies: if you're going to do that, then commit to it. Kill every single last one of them, and like in Nocturne, each monster has a personality. You can talk to all of them, and you can find out what they want and help them get it, and while in Nocturne that's to get them to join your party, Undertale wants you to stop the fighting. If you're going to kill them all because you can, you'd better feel it.
And while Nocturne's True Demon ending is set up with the vague expectation you're probably not going to replay after it, Undertale changes the True Pacifist ending after you've unleashed the demon that comes when you call its name. You've already shown 'I can, so I will' is good enough for you, so in the epilogue you again kill them all, just now it's after they've finally gotten to the surface.
No, player, you are the demon.