I'm sure they attempt to represent the majority, just like ISIS claims to represent all of "true" Islam. But the fact remains that they
don't.
Statistics show that most Muslims don't support ISIS: http://www.pewresearch.org/fact-tank/2015/11/17/in-nations-w...
Even in Saudi Arabia, support for ISIS isn't universal. According to this ( http://www.washingtoninstitute.org/policy-analysis/view/isis... ) there is only 5% grassroots support for ISIS there. Now, even 5% is certainly worrying, but that's nowhere near universal.
Sorry, I remain unconvinced by your arguments.