They are doing this dirty stuff, you are right. I understand they just released an execution video:https://news.vice.com/article/islamic-state-releases-new-footage-showing-scenes-from-massacre-of-1700-iraqi-troops
My only point, is one does wonder some, when they show them lining up say, 16 people on their knees, why those on their knees are not struggling more. But ex-military are going over to fight ISIS, so they certainly are convinced.
220,000 are documented to be killed in Syria since 2011, the breakout of hostilities. So, that is a ton of people and probably about 6 million refugees. There’s no doubting this.
One explanation I’ve heard is that the victims may be heavily drugged and incapable of making a scene.