Oh man, you know what gets me the most out of everything I’ve seen in Undertale so far?

…The way Undyne reacts to figuring out that the player had killed Papyrus, in a Neutral Run.

Especially when you take into consideration what we know about their friendship…

You learn from other run-throughs of the game that Papyrus and Undyne are actually pretty close friends, even if they started out as Mentor and Pupil. It’s most obvious how they get along by calling both of them during a pacifist run.

On the surface it looks like Undyne likes to pick on and mess with Papyrus for her own entertainment at his expense due to his naivete and good nature (and to a degree that’s definitely what she does), but she never does it maliciously in a way that’d hurt his feelings.

Deep down she really cares about him and his well-being, even if she started out being annoyed by how upbeat he was. She came to realize his personality and drive were genuine, and agreed to train him despite him showing up at her door in the middle of the night. She kept all the presents (wrapped bones) he gave her despite her having no use for them whatsoever so as not to hurt his pride. She even enlists him in the help of cheering up Alphys.

She acknowledges that he’s tough and that he could definitely hold his own in a fight but doesn’t train him for the position of Royal Guard because she knows he’s too innocent and nice to go up against someone with the intent to kill.

She knows that if he were to fight someone who wanted to kill him, they’d rip him into “smiling shreds”, because she knows he wouldn’t fight back with the intent to harm someone.

... And that’s exactly what happens in both the Neutral run (if he gets killed) and the Genocide run, and she knows it.