I've never heard of an Islamofacist sending a warning in advance of a mass murder. It's probably some nut job liberal over there "expressing" himself.
I would suspect this was a spur of the moment thing by some Muslim who wanted to vent but didn’t have his bomb-vest ready yet. If nothing else, it shuts down a major tourist site, costs the (evil, Western, infidel) owner a bit of money, and gets publicity.