The news that the murderer of 77 people in Norway has been declared "insane" is not surprising. After all, he was white and not a believer in some scary foreign religion - if he was "sane" he wouldn't have done what he did.
The psychiatric report concluded
[...] he lived in his "own delusional universe where all his thoughts and acts are guided by his delusions"
Okay - this has probably been translated from the original Norweigian, and may well have lost some nuance, but let's break it down:
- His beliefs about the nature of reality were inaccurate
- His thoughts and actions were based on his beliefs about the nature of reality.
The second part of that is essentially what everyone does.
So the "insanity" must be concluded from the first part - that his beliefs about the nature of reality were inaccurate. But this is true to at least some extent of everyone, too.1
Furthermore, plenty of people share his particular belief that there is a Muslim invasion of Europe planned and that our governments are complicit in it. Most of them do not commit mass murder as a result, however.
So: the Norweigian murderer has a commonly-shared mistaken belief, and chose highly illegal actions as a consequence. He therefore cannot be held - by "insanity" - to be criminally responsible for those actions.
Meanwhile, for less default-y terrorist suspects - black, Muslim, non-European, etc. - the idea that they could rationally decide to kill people (even if the decision was based on faulty premises) is completely accepted, and they're tried as criminals.
The idea of that form of "insanity"2 being treated as in some way excusing him of responsibility is completely wrong.
1 For an entirely uncontroversial example that affects almost everyone: optical illusions. They're artefacts of millions of years of evolved visual processing where an "optimisation" that works most of the time gives the wrong result in a few cases.
2 Using the definition of "mistaken belief about reality", anyone who had no reasonable way of knowing based on their perceptions that their action would have a particular criminal consequence should not be held criminally responsible3 - and most of our laws recognise this fact: there are exceedingly few "strict liability" laws.
3 I'm aware that, given what we know about rapists, many of them would be able to use this "defence" on the grounds that they are also deeply mistaken about the nature of reality. However, the precedent that people whose mistaken beliefs make them a general danger to society - highly rare among those considered "insane" by psychiatrists - may be separated from it for as long as those beliefs remain is well-established, and would have much the same effect. At any rate, if we get to the stage where accused rapists are advised by their lawyers to plead "not guilty due to insanity", we'll be at a stage where rape culture is generally considered a "mistaken belief" - and, minus the ablism in that sentiment, I'd be happy with that.