"...the hostage taker will not be even thinking about shooting the hostage if you shot him in the head."
The problem is that there can be any number of non-voluntary physical actions following a head shot, including contractions of muscle groups.
Want to find out if that includes pulling the trigger?
Exactly what will happen to the neurological system and the muscles is not consistent.
One of the signs of brain death is the 'cerebrate disconnect' posture.
The person looks like they have 'put there hands up' as they lie there.
Hands may be clenched, but not always.
It means the cortex is not in control anymore, but the muscles work just fine.
This is not directly applicable if you hit the medulla, but no doctor is going to place a bet on the immediate repsonse of the muscle groups in the body.