Whilst nothing is certain at this stage, it seems as though the massacre in Norway was perpetrated by an extreme nationalist, Anders Breivit.
While a movement or political grouping shouldn'tbe discredited simply because someone like Ander Behring Breivit expressed admiration for or approval of them this will inevitably happen to some extent. His obsessions seem to have been immigration, Islam and multiculturalism. I hope very much that this does not make it harder to point out the flaws and failings in any of those subjects, althoug it will.
The one aspect of his online writings which I do hope become discreditiable after this is his tendency to believe that the other side is engaged in some kind of plot to destroy civilisation. In Brevit's world immigration to Europe wasn't being encouraged because the left believed that it would benefit both Europe and the immigrants but because they hated in Europe and were determined to destroy it.
It is perfectly legitimate to argue that policies will have catastrophic results but believing that your opponents are actively trying to destroy civilisation is usually paranoid nonsense and is not going to lead to anything good.