There is no correspondence of Norway's "Right" to the American conservative right.
Norway (and Europe) has far left international Socialists (communists or Marxists as you prefer), and not so far left National Socialists who are SOCIALISTS to the core, but will not be ruled by those outside or from outside their country. This terrorist is a member of this second group who are Nationalists and Socialists, but are nowhere near a conservative right.
Now that we have cleared up the misapplication of labels (a standard leftist tactic to control the discussion), we can rephrase the question properly, to wit:
Do I believe it is *impossible* for someone who is a National Socialist, to do such a thing?
No, I believe it is not *impossible*, and has occurred historically.
Do I think someone from the American conservative right could do such a thing?
It's possible but much less likely.
Generally, unprovoked mass killings point left while the American right usually requires previous unprovoked killing to retaliate for.
Pretty much the Norway evidence points to cunning National Socialism, and nowhere near modern Christianity or conservatism.
So your argument is, the, that Breivik is a Leftist, because you believe him to be a Nazi? But he killed a camp full of Leftists, didn’t he?
Perhaps these “Left” and “Right” distinctions aren’t holding up very well in the 21st Century. For the second half of the 20th Century, both Nazism and Fascism were viewed as ideologies of the Right. Nowadays the argument is made that Nazism, on the basis of its name and its mass nature, was an ideology from the Left.
How do you classify Fascism? Is *any* mass movement now considered to be from the Left, because its is based on power from a united people? That would only leave Monarchism and Dictatorships on the Right - and perhaps Anarchists (who, interestingly, have usually been considered to be Far Left.)
Since Plato, forms of government had been classified by the nature of the group holding power. Marx added the element of control of the means of production. Do any of those categories of political philosophy enter into your thinking?