Maybe this mental illness explanation is not such a bad idea after all.
They can lock people up for being mentally ill, after all, can't they?
So if we assume that no sane person would massacre a group of children, or cut the testicles off their victims and stuff them into their victims mouths, as happened at Le Bataclan, and if we rightly don't want to demonise millions of peaceful people who happen to have been born into a religion which they can't leave because they'll get killed, then just call all Islamists insane and then they can be locked away in Broadmoor and the world becomes a safer place.
And stop destroying secular governments to the benefit of 'moderate' headchoppers, of course.