Yazidis and most certainly Kurds might be classified broadly as Muslim, but they have been persecuted by their fellow muslims for centuries. Kurds in particular were forced converts to Islam following defeat in battle - interestingly, even following forced conversion to Islam, the cultures remained different to core Islam, maintaining many of their cultural ways, however, this tells a significant tale about what the ultimate goal of the Caliph is all about - groups forced to live under Islam control - unless there is complete surrender and complete adherance to Islamic custom, these groups will always be classified as different and therefore as enemies. Look at the Sunni v Shia and all the sub-sub-sub-sub fighting.
If on the one hand one wants to defend Islam by claiming that ISIL does not represent true Islam, one must therefore define what typical Islam is - but that is where you hit a brick wall - so one can only really hope to define it in broad terms - you cannot therefore dismiss groups like ISIL as un-islamic, because you'd find yourself making excuses for hundreds if not thousands of different groups (all of whom are Islam) whose Islamic beliefs are simply further defined by the radical Imams they follow.
