" I think I’ve hit upon a Doylist explanation for whyUndertale is so morally bizarre:
All the characters in Undertale have no canonical existence, they have all been preemptively rewritten as the characters that fandom would have turned them into.

Undertale as it exists now, is like the fanon version of a game that never existed.

Let’s call this hypothetical game-that-never-was “Undertale Prime”.

In Undertale Prime, Papyrus is pretty much an exact duplicate of Skeletor: an evil mastermind whose plans never come to fruition. Constantly frustrated, taking out his anger on his minions in the most hilariously melodramatic ways.

In Undertale Prime, Undyne is a deadly serious super-soldier. Even a bit of a sadist. She is acquainted with Alphys, but there’s no romance between them.

In Undertale Prime, Mettaton has no Mettaton EX form. He remains a rectangular robot for the entire game, but his personality shows small signs of the sass and flamboyance of Mettaton EX.

In Undertale Prime, Alphys is a tetchy mad scientist, more like Cumberbatch’s Sherlock than anything else. Prickly on the surface, lonely underneath. There’s no mention of anime or internet arguments or anything like that.

In Undertale Prime, Asgore is stern and serious, and completely in charge, but tormented by the necessary evils he has committed to protect his kingdom. Like a more sympathetic version of a king from a Shakespearean tragedy.

And finally, in Undertale Prime, all bosses are killed without remorse or punishment."

"We’ve seen these character archetypes before, and we can guess how a typical fandom would reinterpret these archetypes:

the Thwarted Mastermind becomes a Bumbling Narcissist.

the Deadly Soldier becomes a Hot-Blooded Blockhead.

the Mad Scientist becomes an Adorable Nerd.

The Geometric Robot becomes a Svelte Bishonen.(look at Bill Cipher fanart)

The Tormented King becomes Sad Dad.

(and the most sympathetic/admirable women become lesbians)

But most importantly, all these villains would become sympathetic.

They’d become comedy relief, or even woobies.

Undertale takes the most probable fanon reinterpretations ofUndertale Prime, and makes them canon. Why are the villains actions treated so cavalierly? Because typical fandom wouldn’t care. Typical fandom forgives villains, typical fandom makes villains cute.

But the discrepancy is this: in Undertale, the characters’ actions all remain the same as they would be in the dark and serious story of Undertale Prime. They play the same role in the plot, they are still Villains. The only things that change are their personalities, and the manner in which they are presented to the audience.

The result is that Undertale Prime makes moral sense, butUndertale does not.

It’s as if the Avengers canonically considered Bucky Barnes a family friend and acted as if the events of The Winter Soldierhad never happened, as fandom wishes it were– But Bucky was still a terrorist.

It’s as if the characters in Borderlands 2 saw Handsome Jack as charming comic relief, the way the audience does– but Handsome Jack was still a murderous psychopath.

It’s as if, in Kingdom Hearts 2, Organization XIII were portrayed as the bickering sitcom family that the KH fandom made them into– but they were still trying to kill Sora and friends.

Every playthrough of the Kingdom Hearts franchise involves killing every member of Organization XIII.

But I guarantee you every Kingdom Hearts fan has their favorite Organization member.

None of the characters in Undertale are “held responsible” for attacking Frisk, because a game audience typically does not hold boss characters responsible for attacking the player. Instead, the audience sees them, through a Doylist/Mechanics-oriented lens, as a welcome addition to the game: a challenging battle and an entertaining character.

Undertale takes the player’s expected affectionate attitude towards bosses, and makes it the “objectively morally right” choice, according to the game’s in-world metaphysics.

Undertale is not just a game that preaches pathological altruism, it is, in itself, a pathologically altruistic text– a text that privileges the interpretation it expects to be subjected to over its own internal structure and logic, and preemptively changes itself to make those expected interpretations into objective truth, even when those changes create plotholes and morally repugnant implications.

A game, suffering to make itself everything the world expects it to be, about a child who suffers to make itself everything the world expects it to be."