There are many who will be furious at the attempt to diagnose mass killers, believing this is a way of them 'getting off' or of eluding responsibility. But since the vast majority of those diagnosed with any psychiatric disorder are not violent, personal responsibility probably still has a role to play, even if psychotic processes can be found in these killers.
If a Muslim man emerged from an extremist sub-culture with a worldview, and went on to murder hundreds of innocents in order to further his worldview, you would rightly call him a terrorist. What you certainly would not expect is for Muslim community voices to shun all responsibility by merely dismissing the terrorist as a lone madman, who acted in isolation and was not part of a wider and worrying trend.