If you think detonating a bomb is required to intimidate the civilian population, you don't understand the psychological ramifications of a beheading
People commit heinous murders (stabbing someone dozens of times/dismembering them/keeping them stored in a refrigerator and eating them) on a daily basis, but it doesn't make the perpetrators terrorists. We're bombarded with news info about them 24 hours a day. I don't think this was calculated to strike fear into the heart of the civilian poplulation. I think the cheese slid off the guy's cracker and he turned violent. True to his radical ideology, he "struck the neck of the infidel" while he was getting his vengeance on someone that (in his perception) wronged him.