Every time somebody claiming to be a Christian and motivated by Christian beliefs does something horrible, other Christians are quick to disavow the connection. I've heard that lame defense too many times, something horrible is done in the name of Christianity and other Christians dismiss it as not done by real Christians, who of course would never do such a thing. All the wars that raged across Europe for centuries, the destruction of aboriginal cultures around the world by colonizing European powers, the many crusades against Islam and the several mass murders of Jews on the way, the molestation of children by people in religious orders, slavery and the slave trade.... nope, none of it done by real Christians.
Nitpicking redefinitions of who is and is not a Christian don't get the church or its teachings off the hook. If people like bin Laden and the 9/11 hijackers are Muslim terrorists, Breivik is a Christian terrorist, by any reasonable definition of the terms. Admit it and deal with it, anything else is simply a denial of any responsibility by the faith community.
Now, if you want to go back into history, and start listing off episodes of despicable acts in the name of the Christian Church.....nobody in their right mind could argue with you.
I could indeed argue that the massacres of Jews and Muslims in the Crusades were not done by "Christians"......but they were done at the behest of the Catholic Church........so I concede the point.
One could argue that the witch hunts were usually carried out by civil authorities, for personal or civic gain.........but once again, the institutions concerned were supported by the Church, so once again, I concede the point.
Certainly there has been much done by Christian priests and pastors that is evil, as in the molestation of children by individuals.....but I am not at all sure what that has to do with the discussion at hand.
As for Breivik, he does not represent any congregation of fellows, any institution of common theological belief, he acts only as an individual, and as an individual he has denied the very basis of Christian belief.....therefore he simply is not a Christian........
He could claim to be an elephant, but if he lacks both the basic characteristics of an elephant, and a organization of others that also claim to be elephants.....then he is not an elephant.