Willi,
There must be a difference between a simple murderer and a mass murderer. One is a Rasha, and the other is a Rasha Merusha.
Actually, the Zohar Hakadosh asks how come it says Baavod Resho’im Rina, while we also know about the Chazal of Maase Yodai Tvuim Bayam Veatem Omrim Shira. It answers that there is a difference between someone who did bad and someone who is thoroughly evil. On someone who is completely evil, there is happiness when he’s punished. Otherwise, we always have some Rachmanus.