"Killing" Napstablook makes the protagonist lose one "experience point," leaving EXP unaffected. ... Sparing Napstablook does not abort the Genocide Route. This makes them unique from every other boss, as sparing any other boss aborts the Genocide Route.

Do you kill Napstablook in genocide?

You'll need to hit Napstablook about 4 times. You can't kill him because he's a ghost but he will end the battle at this point. Napstablook's attacks are in the form of tears.

Does the dummy count as a kill?

No. Neither the dummy nor Napstablook count. It is faster to use Act->Talk on the dummy, and to grind all encounters in the three rock room before Napstablook: if you have already exhausted the encounters in RUINS by the time you meet Napstablook, he will simply fade away.