This is as stupidly tactically shortsighted as it is hideous.
Employed against the opposition military, it makes it harder to capture prisoners for intell if they know this awaits them. Harder fighting at a greater cost to you, less surrender.
Employed against non combatant civilians, for everyone you intimidate, you create a mortal enemy from another and their kin.
IEDs/Torture, etc. the tactics of psycho cowards. Stand up and engage mano el mano, oh that's right, you can't, you die at the hands of your betters.