Board Thread:Questions and Answers/@comment-33450064-20171022193730/@comment-30490101-20180103154638

Technically, Undyne- the other Genocide boss- dies in one hit as well. More on that later. I believe there's text somewhere in-game that states that a human with killing intent can easily kill monsters, as their souls are vulnerable to that kind of emotional state. That's why every boss or mini-boss in Genocide dies in one hit: the protagonist really wants to kill them.

Sans, of course, only has one HP so he'd probably die in one hit anyway. His real power, which is why he's the boss of Genocide, is his KARMA and his overwhelming speed. With the two of them combined, it's easily possible to lose 30+ HP in the span of a few seconds. If *cough* you're caught completely off-guard like I was.

Now, as for Undyne, she does die in one hit. She actually starts to turn to dust. The only thing that keeps her alive is, supposedly, Determination. It's this Determination that allows her to actually reform herself into something so powerful she can't be taken down in one hit. Her will is so strong it literally keeps herself from dying, which is why, once she becomes the Undying, it takes a dozen or so hits to finally kill her.