I'm gonna be Captain Obvious here and say that was a fucking brutal beating. That's how you properly beat a motherfucker.
They have no choice in the matter. They're soldiers who are given orders to torture or kill someone and if they don't do it, they will endure what he did and probably worse.MeThinks the idea is to dissuade others. They take a look at the poor dead bastard and decide they don't want to endure what he did.