What will those assessing Anders Behring Breivik probably be looking for? His lawyer has suggested that he's insane, and that he was on drugs. But then, he hasn't many alternatives in trying to defend the indefensible.
There are extremists with religious or political views that go far beyond those held by most people, and which absorb them to the exclusion of almost everything else. But they've not lost touch with reality, and would be considered eccentric and obsessed rather than psychotic. They usually would be content to ruminate on their views, rather than to take direct and destructive action to try to implement them.