If a hanging is done properly, the body drops, the rope plays out, and the noose snaps the neck close to the skull. Death may take some time, but if it's done right, there's little chance that the condemned is awake and aware while dying.
According to at least one account, the U.S. Army hangman (obviously a fading MOS by then) at Nuremberg told folks later that he was so offended by Julius Streicher's defiant cries of "Heil Hitler" on the scaffold that he intentionally shifted the noose a few degrees. As a result, Streicher's neck didn't break, and he died, while conscious, of strangulation.
He had killed before:
"Angel Nieves Diaz, 55, was convicted of fatally shooting Joseph Nagy during a robbery he committed with two other men at the Velvet Swing on Dec. 29, 1979.........
In 1978, Diaz was sentenced in Puerto Rico to 10 to 15 years in prison for the second-degree murder of a prison drug rehab director. Diaz stabbed the man to death while he was incarcerated for armed robbery.
He escaped custody a year later and fled to Florida. In 1981, he fled the Hartford Correctional Center in Connecticut by holding one guard at knifepoint while another was beaten as he and three other inmates escaped."
http://news.yahoo.com/s/ct/20061212/cr_ct/deathrowinmateangeldiazsettodieforslayingmanageroftoplessbar