Moral high ground during WWII was strictly circumscribed. This was how millions of German and Japanese civilians ended up dead while their cities were firebombed. Japan was nuked. Unrestricted submarine warfare (Operation Starvation) in the Pacific was meant to starve Japan into surrender. Ultimately, ISIS's Christian adversaries are struggling to survive against an enemy that, without provocation, has set out to exterminate them. The fact that they are fighting this kind of defensive war means that whatever their means, they have basic morality - the right not to be killed simply for existing - on their side.
Further, unlike American troops, these men don't serve tours of duty, then go back for months of R&R. They are in for the duration. They don't get internet service, all the movies they can watch, hot chow and cold beer on tap and air-conditioned quarters. It's misery on the front lines 24 hours a day, 365 days a year, until they are killed or maimed. I'd cut these men a little slack.
The bible doesn’t.
The sin is forgivable, but (as with any sin) never excusable.
Military action is appropriate. Individual vengeance is not. And if you are going to join the ranks of those who behead your enemies, don’t call yourself a Christian.