Well then, seems gong to war in general would be an attractive pursuit of a psychopath. I once read on Dr. Hare's website that there are individuals who join the army specifically for the opportunity to kill without the usual social consequences. Psychopaths have no internal prohibitions/inhibitions. They chose their behavior purely on the presence or absence of external punishments (and internal and external rewards).
Kadame,
I really think ISIS is par for the course in a war zone. There is a lot of propaganda from the west about them. But I can assure you, the US military also do saw-off heads and worse to people in a war zone.
If you've ever met a marine in person, it's almost a requirement to be a psychopath. One could say the same for the other uniformed branches like the army to an extent. But marines stand out for taking pride in being well trained killers. The more up close and personal personal the better.
They rarely document it themselves and when they do, they are court-martialed. That is the only difference. I think ISIS enjoy unlimited freedom to share their exploits.