Which goes to show that some people, maybe a lot, believe what they want to believe. Virtually every IS image I've ever seen has shown them masked - maybe the beheadings aren't happening, maybe they're all agents of the west, maybe it's all a plot to demonise Islam...
You only have to read and/or listen to the views of some people to realise that their minds are already made up and nothing we can do, no amount of reasoning or education after events such as this will alter that fact. These people are impervious to reason and are the ones who could well be joining the ranks of the extremists.