Mutilating dead bodies is not bravery but a jahilia practice. I am very well aware of the Taliban getting tortured to death by Pak army guys, it put question mark on their professionalism. Taliban are 'bad guys' because they do gruesome things , but if Pak army guys have also the same level of blood lust or even more (for Taliban dont torture their prisoners to death but do quick beheading), then they are also are not exactly good guys. In the end ,the actions of both will be judged by Allah on the judgement day , and only he knows whether actions of Pak army soldiers like those of NLI are in the accordance with Islam or not.
I am not convinced that they can 'terrify' their enemies, they are small with 'cute' and innocent faces. I think due to the terrain, they have high stamina and endurance, and its their fierce loyalty which has always impressed their masters. They are the best 'mercenaries' in the world.
In his story , he found a sleeping Taliban in a tent and beheaded him. Most likely he beheaded an innocent Afghan shepherd , they all have beards. Taliban fighters just dont sleep on the ground in open in the vicinity of British camp. That Gurkha was removed from the service for a very good reason, he committed a war crime.