Board Thread:General Discussion/@comment-29546017-20160809203518/@comment-27701762-20160809204214

I'm not entirely sure what the canon explanation would be.

What immediately comes to mind is that with Napstablook you never inflict any harm on them, which as Sans says is what execution points are meant to signify. Whenever you attack Napstablook, they decrease their HP so that you don't feel bad.

In contrast, with Mettaton, you destroy the body that they love. So while you aren't killing the ghost, you are "killing" the body, so to speak. And that "killing" is inflicting pain.

That's the best I can come up with.