According to Wikipedia, the perpetrator of the Oslo atrocities was involved in mainstream politics within the Norwegian Progress Party, letting his membership lapse in 2007 (presumably at the age of 28.) So this was a man who started out within the political mainstream, and rejected that approach.
Undoubtedly, there will be calls for "lessons to be learned", and stupid new laws to be passed, or new databases to be compiled. If these calls are genuine, what is the appropriate action they should recommend? Two approaches make a little sense: to profile blog commenters, or to profile ex-party members. The second approach makes more sense. The population is far easier to identify, they are clearly a more motivated group, and have made a clear statement by leaving a circle of friends.
So honest authoritarians will be calling for "someone" to profile ex-members of political parties. Ex-councillors who let their memberships lapse are especially suspect.