AFAIK Brevik wasn’t found to be insane and not that many people think he is insane. I’m sure the Norweigen government would like to have him declared insane. I suspect they don’t want other people to do what Brevik did and if Brevik is labelled a nutjob then he can just be written off.
Again, read up on this.
One team of shrinks found him to be paranoid schizophrenic, and psychotic at the time of the murders.
The other team found him to be less mentally ill, but still mentally ill, apparently only with a narcissistic personality disorder and possibly paranoid psychosis.
The courts went with the 2nd diagnosis, finding him sane and able to go to trial.