Religious fanaticism or mentally ill

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The person behind yesterdays terror in Norway was a Danish citizens who have converted to Islam and turned into a Fanatic Islamist.

Today I have read comments like

It has nothing to do with Islam he was mentally ill
and others It has everything to do with Islam and not that he was mentally ill.

So was it religious fanaticism or mentally ill that was the cause of this persons acting ?

I'm convinced it's combination of both.

(To the moderators-This most likely belong to our terror thread)

Markus