The idea that right-wing extremists such as Pierce, McVeigh and Breivik are simply the Christian version of Osama bin Laden is entirely wrong, in other words. Islamist terrorists take (misguided) inspiration from their religious texts in the act of slaughter — explicitly linking their motivation to religion. Mass murdering terrorists with a Christian background (and this includes the IRA, incidentally) typically do no such thing, even if the religious-inspired themes of martyrdom and purification tend to animate their doctrines. Not that this makes mass murder any less hideous or destructive — but it does show it to be a different kind of animal.
I would also add that a big difference between Breivik's attack and those of Islamic radicals is that he operated alone and was almost universally condemned. Islamic radicals work with a huge network of support and when they are successful in their attacks, their supporters dance in the streets of many Muslim cities. Of course, I expect the left to neglect that salient fact.
H/T: The Black Kettle