Race, I believe Nick was already dead at the time of the beheading. There was some apparent editing toward the end of the tape. As fas as Nick being MM's man in Iraq, that could very well be possible, he was, according to his dad, his murderers best friend they could ever have.
I have reviewed the tape frame by frame. There were 2 video cameras used to tape the beheading. Thus the "editing". Nick Berg was absolutely not dead at the time of the beheading depicted on tape.