“The Yazidis practice a Zoroastrian mix of religions that includes elements of Christianity and Islam that ISIS found especially abhorrent...”
Remember, the Yazidi religion is one of the three religions mentioned by Mohammed as deserving a separate status from simply “infidels”. They are considered “people of the Book” (the Book being Torah/Bible), alongside Jews and Christians. So what ISIS would do to Yazidis, they would have no problem doing to the other “People of the Book” as well.
Aren’t the Yazidis devil worshippers? The idea being God is good, won’t hurt them, but the devil on the other hand would.