1 - some murderers are practicing christians/jews/muslims. ISIS do pray.

2 - if they've followed initiation rites (baptism, circumcision, what-have-you) congregate regularly with fellow religion members and do what the religion tells them to, they can be considered to be members of that religion. Being evil does not exclude them, and, more importantly, not following every rule does not exclude them. If it did, there wouldn't be a lot of actual true believers.

Heck, even popes sometimes engaged in depraved criminal activity.