Ha well I think 100% of people face death, just sometimes it comes while you are sleeping and old.
The leaders of ISIS far from only understanding force are using brutality intentionally to draw out anger and a massive campaign against them from the US & NATO. They want to be martyrs to bring less aggressive Muslim nations into the war in support of their deaths. It’s “Rope A Dope” tosucker in the enemy and ambush them with a larger force. Spread the war farther and wider and make your enemy spread it’s resources. Russia has the same plan btw, draw your own conclusions.
Savagery is an old tool used from the beginning. Many many cultures used pain and suffering to control other groups, so this is nothing new. It works best when the perpetrators have the upper hand in the level of force. Against a superior force savagery instills fear but not enough to keep the savage from getting wiped out. For example the American Indian tribes of Sioux. They make ISIS look like punks when you compare what ISIS does as torture, to the the horrid things done to a live human body by the Sioux. The Sioux all ended up on reservations starving after a few years. Somebody had mercy on the remaining Indians because total genocide was on a lot of minds. Every empire of the ancient world used cruelty against it’s opponents in all hemispheres.
Will the guys in a B52 dropping “daisy cutters” or an A10 with cluster bombs be scared of ISIS? I doubt it.
On a local level in the US it’s hard to say how barbarism would manifest itself. All though I think savage behavior would bring groups together to wipe it out.