Board Thread:Questions and Answers/@comment-28050645-20160325024144/@comment-5956954-20160325024508

Killing Napstablook is impossible. When you deplete his HP, he will say he just did it on purpose because you can't kill ghosts, and then you lose one experience point (different from EXP).

When you get "but nobody came" and go genocide mode before Napstablook, finding him in the room that way is ineffective either since he just disappears.