Simon: The issue of who follows the teachings of the Quran most closely is very different than being a "good muslim". I think it is uncontroversial that ISIS is more closely following the Quran than Majiid if you look at the rules in Sharia and check off boxes, but I don't think that is the same as being a good Muslim.
I don't think Islam (or christianity for the matter) is consistent. For instance I think it is inconsistent to say that you are a religion of peace and have the laws and rules in Sharia. If we accept it is not consistent that means you can interpret it in different ways depending on what teachings you accept as literally true and which you interpret to make stuff fit. For instance a person can say that he begins with the "religion of peace" and then remove all the things that don't jive with that using the same methods that modern Christians do; methods for interpreting the crap out of the Islamic texts have been around since the middle ages. Does any of this make sense in an objective way? not really, but none of it makes any sense because it is build on false premises...
Since there are no truth claims at the bottom of it all I don't see why we have to say that person is not as "good" a muslim as the crazy guy in the desert with the AK47. Do we even know if the texts in the Quran has been faithfully transferred?
You can find lists of things Muslims object to that ISIS do. Some items on those lists seems like theological waffling, some seems more concrete. My point is that I think it is both more correct (and a heck of a lot more useful) to either accept that the term "good muslim" is vacuous, or at least that it admits a definition which allows a person like Majiid to say he is as good a Muslim as anyone.