On my way back from the closed library I was attacked by 3 terrible beasts. Fluffeh pugs. Their master commanded them to come home to get treats but the one named Sumi got stuck in the snow. I reluctantly dug her out and she was happy. Not such terrible beasts maybe?
Anonymous undertale theory I found online by anonymous
Combined with Fridge Horror — Why is Mettaton NEO so easy to kill? Because he was meant to be a HUMAN eradication robot. By the time you reach him, you're not really human. Speaking of which, that's why Sans gets the first turn in his boss fight, that's why he is able to dodge attacks when nobody else could, that's why he is able to fake mercy you, and that's why he is able to pause the fight just so you can't kill him. Think about it — you always get the first turn, you're able to dodge attacks (heck, it's even possible to beat Sans without getting hit ONCE), you're able to make monsters sparable and THEN kill them, and you're able to pause the fight so the enemy cannot attack you for a short span of time until you do something. Yup. At that point, Sans is more of a human player character than you are. You aren't fighting Sans. Sans is fighting you.
That's why the Song That Might Play When You Fight Sans doesn't play in the battle. It's Sans' battle theme, a supercharged remix of his Leitmotif, including elements of Papyrus' battle theme and Gaster's theme. Instead, the admittedly awesome track Megalovania plays. Sans is fighting you. Megalovania isn't histheme, it's YOURS. YOU are the Final Boss and he's trying to stop you from triggering The End of the World as We Know It.
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another one...
During the final boss of a No Mercy run, after you finish off Sans, he'll ask Papyrus if he wants anything from Grillby's, which would be incredibly odd considering that to get the No Mercy ending, you'd have to kill Papyrus. Why would Sans do this knowing his brother is dead? Simple, it was his way of peacefully accepting his fate. He said that he knew he couldn't beat you and his last resort was to cause a stalemate and prevent you from attacking; after all, with your shiny new MURDER WEAPON and high stats, you're basically an unstoppable force of evil that murders all in its path. So after he gets hit and starts to leak all that ketchup he drank, he realizes he's taken a mortal blow and limps offscreen so you don't see him slowly die. Him saying "Papyrus you want anything" was his way of finally giving in to his approaching death by mentioning his dead brother in the most calm way possible so he remains just as relaxed and cool as he was the entire game. Addressing Papyrus, whom is clearly dead, is him coping with the fact that he failed to stop you by talking to his deceased sibling as if he were making his way to the afterlife to join him.
This, along with how he hobbles offscreen after defeat, is his way of making sure Frisk/Chara (who are, at this point in the game, a sociopathic mass-murderer) doesn't get the satisfaction of watching him suffer in his final moments. This fits, since the entire point of a No Mercy route is that it's unsatisfying and dull, with your only victories being hollow at best. Sans is by far the longest and most unfairly hard battle in the entire game, and when you finally get a hit in, he doesn't cry, lament his defeat, or drop his permanent grin. He just says "don't say I didn't warn you", climbs to his feet, and calmly staggers away, acting like you didn't just hand his ass to him.
During a No Mercy run, Asgore asks "What kind of monster are you...?" The obvious implication is that he's briefly mistaken you for one of his own kind, since it's commented several times throughout the game that you've ceased to appear human. The less obvious one is that this statement is not addressed to the character.
This is played with both ways with respects to the mirrors in Toriel's home ( which gives the dialogue message "It's you!"), and New Home (which, on a pacifist run gives the dialog "Despite everything, it's still you."). On a genocide run, however...
In addition, after the Fallen Child is implied to kill you, it doesn't really add up that they can do that much damage when the main reason they were slaughtering people was because of how monsters worked. Then again, you kind of are a monster for taking this route to start with.
Alternatively, at this point they're wielding an actual knife, and not an improvised weapon, so if they attack you, a human, it would definitely do fatal damage if they struck something vital (hence the infinite 9s), and given what a psychopath they are, the odds of them knowing where to stab to kill are pretty high...
Though the Fallen Child's intention was to destroy the world. It would take an absurd amount of damage like that to destroy the world, and as a result there is only blackness afterwards, even when the player returns.