He invited the Demons of Islam to come in.
We wrestle not against flesh and blood enemies, but against the powers of darkness. The "anger" and whatever leading up to this guy's "conversion" made him a ripe target for the cult of islam. What we call "Mental Illness" is more than just a misfiring of synapses or a chemical imbalance in the brain. This is a monumental struggle between GOOD and EVIL, folks.
As the writers of The Dead Sea Scrolls called it "The Epic Battle Between The Children of Light and The Children Of Darkness".
I don't make this stuff up.