Three posts on 4chan predicted the massacre. Motive as stated was so that metal detectors and backscatter machines would become mandatory first in casinos and then in many other venues. Ground level investors in the two companies involved [Chertoff was one, as I recall] would walk away with billions.
http://truthfeednews.com/wow-ominous-message-predicting-vegas-massacre-posted-days-before-event/
I was surprised by how quickly the MSM picked it up (though not about the instantaneous gun-control bent), but that is even quicker.