Mankind has probed the limits of cruelty but the boundries have yet to be defined. throughout recorded history you have Romans enjoying the public spectacle of humans being mauled and killed by wild animals, entire Chinese armies that were captured slaughtered by their victorious captors, Mongols surrounding huge area and killing every human within the circle, the systematic, efficient , sadistic Holocaust perpetuated by thos “cultured” Germans, the less efficient but equally brutal Rwanda genocide, and of course the recent barbaric actions of the jihadists who believe heaven awaits them as a reward for their crimes. The limits of cruelty and barbarity have not yet been defined. Woody Allen said it best. When accused of not acknowledging the enormity of the Holocaust and being a self hating Jew, he responded that there is absolutely no doubt that six million Jews were murdered, but never forget as well that records are meant to be broken.
...How could such pre-modern psychopaths ever be defeated...
Lots of things can be done when field commanders have discretion rather than politicians running the battlefield.
Now we should go after whoever funded, trained, and sustained ISIS during their brutal rampage, and visit some destruction upon them too.