With a handgun, this is a very touchy situation. Handguns are notorious for acting strangely in and around the skull. IF proper penetration and aim can be assured, the desired disconnect between the head and everything below can be achieved. (Maybe with one of those magical Nytrillium bullets.)
This kind of shot can be made with a decent caliber rifle, both from the increased accuracy angle, but also from the increased energy to plow reliably on a path directly through the brainstem.
A brainstem disconnect will do what is desired. Being there with the skill and power is the problem. Done correctly, there will be no twitches or other striated muscle tension.
I know that John Farnam has been looking into brainstem targeting, but I do not know if he has incorporated it into his training, or not.
Hits that do not disconnect the electrical system at the brainstem are a crapshoot. I've seen a few head shots with handguns, including one that was dead center between the eyebrows. This guy was walking around and talking to people like nothing had happened. The hole appeared to be about 9mm or .38 cal. Unfortunately, I never found out the actual path of the bullet. Others ceased conscious activity immediately, but not all of them.
Personally, I wouldn't risk it unless there was no doubt that the criminal planned to kill his hostage within the next few seconds.