from my humble point of view
whenever you have a scared or holy (ideology ) where people follow the prophet of this ideology and consider him the perfect human who don't do mistakes and should be considered and copied as a role model ( then this role model should be moral and ethical enough other ways we might end up with 2 billion people who are confused by moral common sense and there perfect human being )
now also when the door for interpretation is open , and historical example of the perfect human being doing similar acts , i can totally understand why would ISIS consider them self as true Muslims , and see other Muslims as confused lost Muslims who are victims of there (personal moral common sense and lack of Islamic history understanding )
now to understand other Muslims who condemn ISIS i will give you example of the people who i know best
(ME two years ago before i renounced Islam , my family , most of my surrounding ) every one i know find ISIS completely insane and un Islamic even i was astonished on HOW WOULD THEY EVEN CLAM THEY ARE MUSLIMS
until i went deeper into Islam and deeper into Mohamed history , you see the Islam i grew up with what they teach us in schools in the capital cities is selected clean , censored , beautiful verses , when ever we question any thing we either get the reply of god work in mysterious way or simply your human understanding is limited and we shouldn't question Allah or his profit if i asked about slavery or women treatment in Islam i get sugar coated answers to numb my anger and stop my questioning .
so i understood that i cant be a muslim after realizing that i cant approve most of the things that i later on knew about the prophet history which no one told me about while being muslim
i used to watch david wood videos on youtube and hear about stuff i never knew before and then as native arabic speaker just go to muslim web site and see how they justify it just to make sure it was true .
so who is true muslim ? I guess those who know islam better , not the ignorant un educated poor who cant read or write and being used for other porpuses , nor the civilized middle class who are spoon fed islam to make it look what it isn?t