I don't get it

I don't get it. WTF are these militants thinking? So your pissed off that the world is against you. So why do they take it out on civillians. Doing so has indeed heightened (sp) the level of fear but doesn't this fall under war crimes? I mean, take it out on military personal. Don't get me wrong, I think beheading someone is sick. Hold them prisoner and if you HAVE to kill them, just shoot them in the head or something. Anything more civilized than beheading. I just keep telling myself, we all reap what we sow.