They created the videos so people would see them and instil fear
When someone bombs a mall or hijacks a plane, that's pretty clear they're trying to create a fear of shopping or travelling. But what is it that the isis team is trying to do with their beheading videos? What exactly are they trying to make us fearful of, getting our heads sawn off? I didn't really need a video to have that already high on my "do not attempt at home" list. Fear of being an aid worker in a combat zone? Anyone who is in Syria and the like as an aid worker has either accepted the risks or is hopelessly naive, so that can't really be it either. So, whatever reason they have, the actual result of showing these videos is to firmly illustrate what complete barbarians they are and to dissuade anyone who might be thinking to just live and let live with these people.