Talk:Genocide Route/@comment-27116270-20160403224654/@comment-27701762-20160404013142

It's not strictly necessary to get the message, but it's the safest way to make sure that you've cleared all of the monsters.

It's possible that you're just miscounting, perhaps by counting a kill that doesn't contribute to the kill counter (for instance, Napstablook doesn't count as a kill).