It's the Quran, Stupid
I've noticed something that most pictures of ISIS beheadings have in common. That is, the murderers hold the knife in their left hand and the prisoner with their right hand. I think goes back to the Quranic description of war booty is that what "the right hand possesses", in this case, the prisoner. It may also reflect on the admonition that the left hand is for "unclean" purposes, such as wiping one's ass.
Since the executions are fraught with symbolism and much of what Muslims do in everyday life is symbolic, this may be why they behead with the left hand. It all points back to being religiously motivated.