We set up a system where society approves of the use of lethal force. (In fact, many believe the true defining characteristic of a government is the monopoly in the use of force) People are instructed by this system to go kill people.
As long as that's all the information we have, there's nothing to indicate that these people care nothing about societal norms. In fact, they might actually be less sociopathic than those who do not serve. The only thing we'rd offered is descriptions of the cavalier attitude they have towards death, but, yet again, this is not an indicator of being a sociopath. Perhaps a callous and heartless person. Perhaps a crazy person. Perhaps as you point out they may actually be sociopaths. But nothing in this article tells us one way or another. Instead we're presented with these experiences as being the "true" nature of the entire conflict, and then the brutal and crass attitudes observed as being indicative of some sort of psychiatric disorder. That's just a little too much wringing of the hands and over-reaching for my comfort.
There is a premise here: going off and sneaking through the high grass to kill somebody in a brutal and bloody fashion without remorse is indicative of a psychiatric disorder. I'd like to explore that idea. But this article doesn't go there. Instead it's trying to be a "yeah, well this is how it really is, kid." and all I'm saying is to take such stories -- no matter what their slant -- with a gain of salt.
If it makes you feel any better, if the author had written the same tough-guy-been-there story with the soldiers all acting like boy scouts I would feel the same way. The only difference is that there would be plenty of folks willing to take that apart for me, so no comment would be needed.