Good luck to the photographer trying to forget what he saw. Sounds like a recipe for PTSD in short order. What's the deal with Arabs and beheading? It seems to be their preferred method of execution. Is there some religious significance to it? Or is it just to create fear among their enemies? There are definitely easier ways to kill someone.
This is what Slate has to say about the subject. I seem to recall a U.S. soldier got into some hot water for taking the head of a combatant (post-mortem) to prove that that particular person had been killed. The way I remember it, was that whoever the local religious leader was, considered that to be a desecration of the body. I searched for the story, but the only things that show up are about that incident where a British soldier was killed in the street. For an intelligent species, we're pretty awful pretty often. Individually, I think we're mostly ok.