You can't deny some a genuine defence because you suspect it's not a genuine defence. That's the whole point of the legal system.
That's why I am interested in what he meant when he said mental health in this case was "very predictable". Earlier in the thread he said:
I would say it is a given that this lad has mental health issues, no one commits a crime like that, even if they think they have some religious or other nonsensical reason to do so, to commit a crime of these proportions against young children can only be committed by someone with something very very wrong with their heads.