It is quite plain what Christians are to think of such actions. The Bible is abundantly clear that "You shall not commit murder." Yet a word is perhaps necessary since Breivik claims to be a Christian.
That profession is quite false, he claims to be a disciple of Jesus, but Jesus "went around doing good". Jesus did not come to destroy, but to save, and in John 18:36 Jesus says, "My kingdom is not of this world. If my kingdom were of this world, my servants would fight." So, then, it is clear that a man who seeks to kill people is not a Christian, and cannot be. No murderer can inherit eternal life unless he first repents and believes in Jesus. Breivik is unrepentant, he is therefore still lost and in the gall of bitterness.
He is described by the press as "a right-wing Christian extremist." He is not a Christian, whatever he may claim, because he ignores the law of God. He is said to believe he is in a war. I have my suspicions that Breivik may be insane, but the reality of the matter is that he is not in a war. He is not a soldier except in his own imagination, and he has no warrant for committing the crime that he has committed.
Christians follow the Prince of Peace. No man does God a service by murder, whatever he may think, and the man who kills in Jesus' name is committing an act of blasphemy as he does so. "You shall do no murder" still stands in the Law of God, and the man who says, "I love God" but does not love his neighbour is a liar.
The message of Christianity is the Gospel, the word means Good News. Breivik's twisted ideology is one of hate, and is good news to none. I sincerely doubt whether he even knows the meaning of the word grace, or the meaning of love. For God's love is self-sacrificial, it is the love of Jesus dying on the cross to take away sin. And there is grace even for Anders Behring Breivik if he repents of his sins and trusts in the Lord Jesus Christ.
He is no Christian who commits murder. But we follow a Christ who was murdered, and who prayed for his killers, "Father, forgive them."