[She'll go ahead and stab him a few more times even so because apparently she and Sans finally have something in common.]

No wonder he's in charge of your sin, he's almost as annoying as you are!

So if we can't kill him, what should we do with him? Feed him to Rahab? Toss him into lava? There's a ton of explosives in the basement, maybe we can blow him up!

[undyne doesn't this all constitute as trying to kill him]