How often did Roman gladiators kill each other?

Gladiators' combat had become a martial art by the beginning of the first millennium, according to a controversial theory based on reconstructing the fighters' tactics from Roman artefacts and medieval fight books.

To amuse the crowds around the arena the gladiators would display broad fighting skills rather than fight for their lives, argues archaeologist Steve Tuck of the University of Miami. "Gladiatorial combat is seen as being related to killing and shedding blood," he says. "But I think that what we are seeing is an entertaining martial art that was spectator-oriented."

Here is the full story.  Here is the home page of the researcher, Steve Tuck.  Here is a page on Bruno Sammartino.