ISIS fighters are a bunch of savages. Once we start engaging in their behavior, we also become savages. Of course the ISIS fighter should be dealt with in an appropriate manner. Perhaps not beheading.
Nietzsche said, "Whoever fights monsters should see to it that in the process he does not become a monster. And if you gaze long enough into an abyss, the abyss will gaze back into you."
thing is, they (the opposition, meaning ISIS) looks down on those who don't "get on their level". They respect those who are willing to do what they do. And I was told by people who went to Vietnam, that the Vietnamese were the same in this regard.
We are all "savages". Every civilization believes that those who attacking them are barbarians or savages---just a name to demean those who deem as beneath us.
Nietzsche probably knew nothing of fighting, let alone war. And as the saying goes, "it takes a bigger devil (Monster) to beat a devil (monster)." So, one has to be prepared to be that bigger devil (monster).