Dude, how can you get this conclusion from what I said? Or is this a strawman or what?
As I said, the definition of "muslim" differs by interpretation because like any other religion, people interpret the text differently. Shia muslims say that they are right. Sunnis think they are right, and within each there is more and more denominations that think they are right and none but them. There is so much infighting between them it makes leftist-infighting look like child's play.
So yes. One of the interpretations, adopted by ISIS, says that these muslims are considered kuffar (people who have rejected god), because they did not seek to learn about their religion and establish the rules that Allah commanded. Even if they were ignorant about their religion, it doesnt matter, they should have learned as it's their duty as muslims. Thus, it is okay to kill them (Cuz yes, the quran is okay with killing and enslaving non-muslims, in fact it explicitly encourages it.) So this is how they justify killing them.
Obviously this is the niche explanation of ISIS. Not mainstream islamic interpretations.
Everything ISIS did is a valid interpretation of quran and hadith. Sunni muslims use two sources to make rulings, one is the qur'an and the second is hadith. Because the atrocities are countless, let's talk about two things they did often:
Slavery, as you see, is permitted in islam and is mentioned countless times in the quran itself. You can look up the verse I mentioned.
Not only that, it is also supported by hadith in multiple Sahih hadith (which is the highest degree of trust in a hadith being correct and traced back to the time of Muhammed by muslim scholars).
The first hadith talks about his wife Safiyyah Bint Huyayy. She was enslaved during the battle of Khaybar, taken at first by Dihyah Al-Kalby. Muhammed then heard about her beauty and status and decided to take her for himself. I would also like to mention by the way, that she lost her husband and father that day, killed in the battle. She was then taken by Muhammed as a slave, and he slept with her on their way back that night. Not even 24 hours after the death of her father and her husband.
So yes. Muhammed himself partook in slavery.
Another one was about Aayisha and how she set one of her slaves free, then told Muhammed about it. His own response was "Had you given her to one of your uncles would have been of higher reward to you from Allah." I don't remember the hadith verbatim, but it's along those lines. Again, I encourage you to look this up.
Verse 4 from Surat Muhammed justifies this by telling those who believe in god that: "if they ever meet anyone who is a non-believer they should hit their necks [with their swords] until they kill a large number of them, then take the rest as prisoners of war, until the war ends, and if god wanted he'd win this war...etc (Look up the verse)."
As for the difference between Makkah quran and the quran of Madina, you will find a great shift of tone from the "peaceful co-existence" tone in Makkah quran, to talking about Munaffiqeen and fighting kuffar in Madina Quran. The difference is crazy that the verses contradict one another. Especially verses in surat Al-Tawbah. Scholars solve this problem of contradictions by saying that the latter surahs and verses (and even hadiths in some cases!) essentially override the early verses (Something called Naskh).
So yes.The actions of ISIS can be justified from the Quran and Hadith. I am also willing to argue that this was the way of life of Muhammed and his Sahabah 1400 years ago. A life of war, genocide, and enslavement, as is supported by countless Hadiths and verses from the quran, of which I only told you very few lol.
Now, again I would like to stress. The fact that they do this against other muslims as well by saying they are kaffir, is as I said, not the mainstream view. In fact, most muslim sheikhs and scholars throughout history took the idea of calling a mulsim kafir as taboo. Because they believed only Allah was allowed to say that about someone. The reason is that by saying someone is kaffir you are essentially saying they are going to hell. Thus, making a ruling that only Allah is allowed to make.
So only minority groups of muslims like ISIS do such thing, while even large institutions like Al-Azhar in Egypt and other muslim institutions refrain from using that language against ISIS.