> ... a barbarity we would never in our wildest, darkest fears thought we'd see ... we're dealing with the most fanatical maniacs of all time.
Don't think so. These people are simply medieval. Joan of Arc, a 19 year old girl, was burned at the stake. And in this country were "witches" who, having survived being drowned, were also burned alive. Not to mention what has been done in Europe within living memory.
How to deal with IS? Obviously military action is required. At the same time, if we play them at their own medieval game then we lose by driving recruits to them, both locally and globally. We have to have as many of those recruits as possible on our side, by showing them that our way of life is preferable and making sure that IS, and only IS, are perceived as the bad guys. If our response to this is that hundreds or thousands of innocent people are killed alongside some bad guys then that would probably not be helpful.