Perhaps the act itself is being used as propaganda. Somebody wants Christians, Jews and Muslims to wipe each other out. They want a religous war.
I find it really, really difficult to think these wild-eyed ISIS types are capable of being secretly controlled by shadowy behind the scenes figures so beloved of conspiracy theorists.
FWIW, Islam has almost 1500 years of such wild-eyed fanatics showing up for a brief period till the exhaust their welcome. Sometimes they even succeed in establishing a State, whereupon they immediately start losing their fanaticism.
Would seem to me that trying to control this sort would be like trying to herd wolverines. Which doesn’t mean they aren’t getting secret support from people who just want to cause trouble. But control? I really doubt it.