You're thinking like them. Perhaps the larger purpose of the videos is to further embolden the West's resolve against such butchery.
I think the purpose of the video is to scare away American workers who are rebuilding the infrastructure of Iraqi. Once the infrastructure is done, free Iraqis will fight on their own to preserve it. The terrorists want to have one of their own sicko mullah powerheads, worse then Saddam in Iraqi, so they can have a new base of operations.
The more they slow the rebuilding process and cause turmoil, the better chance a terrorist regime like Iran can begin installing mullahs instead of democracy. Now that we are in, we must win.