Asgore killed six humans (whether or not they were all children is left open, but it's strongly implied that they were) because he swore he would give his people hope, and only the world-ending promise of breaking out of the Underground and taking revenge on humanity was powerful enough to rekindle the hope extinguished by the deaths of Asriel and Chara. Was it only that which disgusted Toriel to the point of fleeing him? I don't think it was JUST that. Toriel herself points out a way that Asgore could've done it that would have repelled her a bit less. Was it because it implied killing lost children? Or was there even more?