User blog comment:B214/Light Novel Volume 16 spoilers/@comment-6096197-20131020081334/@comment-6096197-20131021143658

Gasper didn't kill all vampire. he just kill valerie brother and his followers.

There was a coup'detat in the Tepes faction. It was started by Valerie's brother and several VIP Vampires that decided to follow him. Those guys were the ones who Gasper massacred.