A case in point is Stephen Prothero writing for CNN's religion blog, who points out that Brevik very certainly professed christian beliefs that went far beyond mere "cultural identity". Brevik, for example, made a youtube video in which he talked about how he "martyrs" like him would "soon dine in the Kingdom of Heaven". He quoted extensively from books of the Bible such as "Exodus, Samuel, Judges, Psalms, Luke, Matthew, Isaiah, Daniel, 1 Corinthians, 2 Corinthians", and cites verses such as "God can be a Man of War if He wants to be".
This is not all that different from right-wing Christian fanatics like the so-called "Army of God" who endorse bombing of abortion clinics and the killing of doctors who perform abortion. They have biblical quotes all over their eponymous (look it up to figure out the url) on their website explaining how it is a-ok to kill anyone who their religion calls an "evildoer", which is everyone who is not a fanatic, bible-literalist.
BTW, this is why it is always funny to listen to religious fanatics who claim to believe in "moral absolutes". They love to claim that they believe in these "absolutes" and everyone else is a "squishy relativist" who thinks that morality is "situational". Then, when they want to justify killing abortion providers and the people who work for them they say, "The bible isn't against all killing. It's just against UNJUST killing". Oh really, so the commandment says, "Thou shalt not kill UNJUSTLY". Or they say, "we are just against the taking of INNOCENT life". Again, I don't recall the commandment saying, "Thou shalt not kill INNOCENT people". See there is always a way that you can say "My kind of killing is the GOOD kind, because it advances MY agenda, but the kind of killing that other person does is BAD killing, (because it works against my agenda)."
See Batsh*t Brevik thought his kind of killing was GOOD killing, and the kind that Muslims do is BAD killing. Anti-abortionists think that killing their opponents is GOOD killing, but anyone who might try to kill them would be BAD. The Crusaders and Knights Templar thought they were killing according to God's will and with God's blessings when they destroyed towns, not just in Palestine, but in Europe and Asia Minor. Saladin thought his actions against the Crusaders were justified by his religious commitments to Islam.
The point, dear friends, is that religion is one of the easiest tools to invoke and manipulate when you want to kill someone else. It's one of the easiest rackets of all time to use to justify just about anything.