Reports are that his captors re-enacted his burning many many times; maybe he thought it was another re-enactment in which his captors would ultimately extinguish the fire at some point - then it was too late.
And it also has been reported that many of these captors have been sedated into some level of submission before execution.
I have always thought the beheading victims seemed passive; almost drugged into a calm submission.
Did you watch the whole video?