If it turns out that he's "a right-wing Christian fundamentalist" and those doctrines provided a motive for his behaviour then such doctrines need to be denounced and combatted. In just the same way that the doctrines that give rise to terrorists shouting 'Allahu Ackbar' also need to be denounced and combatted. Of course, that would expose a contradiction at the heart of our society - or, perhaps more accurately, a self-hatred. Muslims etc are victims of the oppressive West, therefore they are on the side of the angels. This Norwegian nutter is an expression of the oppressive West, therefore he is on the side of the devils.
Perhaps there are all sorts of mitigating circumstances and doubtless we will come up with all sorts of explanations but in the end evil is as evil does and he is responsible to the Almighty for what he has done.
Why, O LORD, do you stand far off?
Why do you hide yourself in times of trouble?
In his arrogance the wicked man hunts down the weak,
who are caught in the schemes he devises.
He boasts of the cravings of his heart;
he blesses the greedy and reviles the LORD.
In his pride the wicked does not seek him;
in all his thoughts there is no room for God.
His ways are always prosperous;
he is haughty and your laws are far from him;
he sneers at all his enemies.
He says to himself, Nothing will shake me;
I'll always be happy and never have trouble.
His mouth is full of curses and lies and threats;
trouble and evil are under his tongue.
He lies in wait near the villages;
from ambush he murders the innocent, watching in secret for his victims.
He lies in wait like a lion in cover;
he lies in wait to catch the helpless;
he catches the helpless and drags them off in his net.
His victims are crushed, they collapse;
they fall under his strength.
He says to himself, God has forgotten;
he covers his face and never sees.
Arise, LORD! Lift up your hand, O God.
Do not forget the helpless.
Why does the wicked man revile God?
Why does he say to himself, He won't call me to account?
But you, O God, do see trouble and grief; you consider it to take it in hand.
The victim commits himself to you; you are the helper of the fatherless.
Break the arm of the wicked and evil man;
call him to account for his wickedness that would not be found out.
The LORD is King for ever and ever; the nations will perish from his land.
You hear, O LORD, the desire of the afflicted;
you encourage them, and you listen to their cry,
defending the fatherless and the oppressed,
in order that man, who is of the earth, may terrify no more.
Psalm 10, NIV
(NB for a flavour of what I'm guessing is part of his motivation, do some research on Fjordman and read his writings. They are very interesting and have been very influential.)