Why do people think the Norway shooter is a white nationalist?

[Q]

Nationalists have been known to kill members of their own ethnic group using the argument that the people they are killing are traitors to the cause of their group. Like, Hitler killed many of his own allies, memers of his party, even members of his inner circle, on the Night of the Long Knives, because he felt they were a threat and killing them would serve the cause. Same could be true of this guy.