I think the thing that makes undynes genocide fight so powerful is she is not only fighting for all monsters, but for EVERYONE.
“This isn't just about monsters anymore, is it? If you get past me, you'll... You'll destroy them all, won't you? Monsters... Humans... Everyone... Everyone's hopes. Everyone's dreams. Vanquished in an instant. But I WON'T let you do that.”
The fact that she was so insistent on hating humanity to switching to wanting to protect everyone, including humans, from you. Her fight is her truly giving every last piece of herself in an attempt to save everyone.
it's funny... somehow genocide gives a lot of the main cast a chance to have the same character development they have in pacifist.
papyrus, after seeing you ignore and ruin his puzzles without even just batting an eye, realizes his dreams of stardom and vaguely defined popularity aren't the right goal, and he should treasure his friendship with undyne instead (you don't even have to mention everything about his "i shall be your friend and guardian" speech before we confront him)
like you just said, undyne gets over her blind hatred for humanity and recongizes humans as the next logical victims of your annihilation, so she fights for THEIR hopes and dreams too!
alphys finally faces her mistakes for the greater good and literally saves half the population by evacuating the rest of the underground. EVEN bringing them to the true lab for safety where she hid the amalgamates, and going around hotland IN PERSON to make sure no one was left behind.
and while with sans it's much more of a grey case, he does finally get the desperation determination to ACT and give his actions some weight, even in spite of the resets.
flowey finally sees the horror of the "kill or be killed" mentality and returns to considering himself as part of the other monsters, and not Something Else Above Them (such as him begging Asgore for help)
it's almost ironic. it's almost poetic.