They would if they had a dream towards dawn, when the evil djin who bring bad dreams have turned in for the night. In the last few hours before dawn, Muslims believe they get "true dreams" from their god (because the evil djinn are tired and have apparently gone to their own beds by then)
So, if the shooter is a devout Muslim who is being directed by "God" - HIS god - then he very well might pass on a message he believes his god has told him to give. Religious wackos are the same all over.