"Then you could just leave! You know how badly we hurt monsters without even trying!" Their heart had never been in that fight with Asgore, not in a single timeline. Only desperation and the knowledge that Flowey had rigged the game ever let Frisk start swinging at all--and every time the boss monster had nearly died at their hands. Someone to great and huge and powerful, defeated a tiny child with a knife.

(Not that they ever felt his dust hadn't stained their soul. It was just like Flowey said--he wouldn't be dead if not for their help.)