I'm increasingly convinced that stuff like this is the wrong way of looking at it. I don't think we're looking at people led through religious ideology to barbarism; we're looking at people who fancy a bit of barbarism but need a convenient faith-based excuse.
In fact, I bet if IS dropped all that religious nonsense and just stuck to the head choppings, immolations, and all the other what-is-good-Conan stuff they'd be absolutely inundated with international volunteers. Basements would be empty, games consoles would go unattended and cats remain untortured throughout the Western world.
Of course, Islam here does provide an interpretive framework through which people can justify their murderous proclivities to themselves and the like-minded and its arguably a necessary social framework which enables IS domination of the territory of people of a shared faith. But from the point of view of international recruitment, all the praying five times a day, malevolent parsing of obscure hadiths and wacky dietary restrictions has to be a drag on the market. It's just not the way to appeal to promising constituencies like, say, the gamergate crowd.