To me she's pretty obviously an on-&-off anti-hero/anti-villain, I'm just making sure since she doesn't already have a page here and I'd rather not make a page just to potentially have it deleted
Although she's pretty much an actual hero to monsters, being willing to sacrifice herself to save monster kid even, she is pretty heinous when it comes to humans, and I'd argue she's overall even worse than asgore, who already has a page here
To the protagonist, a human child, even if they have not killed anybody and even if she witnessed them go out of their way to save monster kid's life, undyne says the following, while trying to turn them into a kebab:
"You're just a coward! Hiding behind that kid so you could run away from me again! And let's not forget your wimpy goody-two-shoes-shtick! Oooh! I'm making such a difference by hugging random strangers! You know what would be more valuable to everyone? IF YOU WERE DEAD!!! That's right, human! Your continued existence is a crime!"
"NGAHHH!!! DIE ALREADY, YOU LITTLE BRAT!"
"YOU'LL NEVER ESCAPE FROM ME!!!"
Quite racist innit?
Even after the protagonist saves her from heatstroke seconds after she fainted while trying to skewer them, undyne still despises them and even briefly still shows interest in getting "revenge" on them after she has a hang-out session with them and gets to know them better
In the asriel boss fight, after you save her, undyne says "Well, some humans are OK, I guess!", which is pretty solid evidence she's genuinely racist against humans before the protagonist fully redeems her lol
In the neutral ending where she becomes ruler of the underground, undyne "expands the royal guard massively", pretty vague, and plans to destroy humanity, except unlike asgore she actually wants to destroy humanity, especially the protagonist, and is taking the task seriously
According to gerson, undyne would attack non-bad guys when she was a kid, like the mailman, but that pales in comparison to everything else and, I mean, she was an impulsive kid
So her crimes are attempted child murder, conspiring genocide and assault