@JemyM
I agree, and for a long time the extreme right wing nationalists were considered the most dangerous even by the general crowd. That opinion seems to have changed recently (mainly due to muslim terrorists), but there might be another shift now because of Breivik.
To understand why an idea might be a catalyst, I think it's important to look whether or not the idea includes a "people".
There is a well-known bias called the actor-observer effect which can almost exclusively be blamed for this kind of reasoning, especially lethal when blended to a false-dichotomy. The Actor-Observer effect means the tendency to attribute our own behavior to external sources and others behavior to the person. The Actor-Observer effect can also be applied on a perceived group. Moral and positive behavior is attributed to the in-group, immoral and negative behavior is attributed to or blamed to external sources (the economy, the whether, a scapegoat etc). The out-group is attributed the other way around, good and moral acts in that group is attributed to external sources where as immoral and negative behavior is attributed to the group.
Now the idea about the "people" is in general a false dichotomy. That is, it promotes the view that there are two distinct groups who are different and stand against eachother. The people can be the French, the Christians, the Workers, the Men. Most of the lethal ideologies included a such dichotomy that created an artificial difference between a perceived ingroup and outgroup with negative consequences. Traditional nationalism is often discussed in relation to the country, with Germany during the 2nd World War. But the same idea of the "people" is just as expressed in branches of religion where as great importance is put on whether or not an individual subscribed to the religion or not. Communism didn't just include the dichotomy but the importance of creating the dichotomy for the ideology to succeed, so we have the Bourgeois and the Proletariat. Radical feminism depends on the idea about the Patriarchy.
In our time, one of the least questioned dichotomy is the one about the West and the East, now usually expressed as the Western Christian Democracies vs the Eastern Muslim Terror. There is a "clash of civilizations".
Many so-called westerners who worked in the middle-east have however pointed out that most of the disagreements within western politics is also present within eastern politics. There are feminist Muslims. There are democracy movements. There are scholars, freethinkers, human-rights activists and HBTQ movements working within what we commonly generalize as "the east". The debate and the challenges faced by these groups are pretty much identical to what was going on in the "west" 100 years ago.
Black & White thinking is notorious for not only creating a clear expressed dichotomy between one group and the other. They tend to place all criticism no matter the source to the out-group. A Norwegian who questions the nationalist movement is just another muslim or is muslim-friendly. Today we commonly see the expression "the cultural-marxists, the feminists, the left-wing, the politically correct" be used in the same sentence, often including homosexuals, the jews and the muslims. All of these are boiled down to the same group; "the enemy".
Personally I am commonly referred to as the "muslim" or said to have "Hostility against Sweden" by members of this reasoning.