Whenever there is an enemy human instinct is to dehumanise and classify those who are against us as some sort of embodiment of pure evil whose motivations are the mirror opposite of our own more noble intentions. Where we hold life dear they deem it to be worthless etc.
But that's too simplistic. In your analysis you have split the enemy into the truly evil and then the deluded followers brainwashed into evil acts by their genuinely evil ring masters. Again too simplistic. More likely is that, with the exception of a few genuine psychopaths, most ISIS members consider themselves to hold human life very dearly indeed and for that to be one of the core reasons for their actions.
No doubt they consider the apparent lack of concern the West holds for Arab lives as a sure sign of our depraved inhumanity and deem this a dehumanising justification for why Western lives matter so little. In their eyes we are the evil 'other' who have little regard for the human lives that they can most about.
No doubt I will be accused of advocating some sort of moral equivalence. But I'm not. It's perfectly possible to take a firm moral position on one side rather than another, to whole heartedly condemn the actions and philosophy of ISIS, whilst simultaneously acknowledging that human motivations and psychological needs are universally human.
In fact I would suggest that NOT dehumanising those we oppose, NOT doing as they do in that respect, is a key part of maintaining a morally superior position.
Are ISIS members motivated to do the things they do by the loss of lives they care about? It certainly seems so. In which case it is not that they hold life cheap so much as they have split the world into the human lives that matter and the dehumanised ones that don't. A very typical human psychological response.
I say we try not to follow that same path.