Whenever someone refers to football players, or extreme fighters, or NASCAR driver as "gladiators", you can show them this, from Mind Hacks (I don't have access to PubMed and so can't excerpt the original article).  It's an examination of skulls from a gladiator cemetery in Turkey, matching fractures with known gladiator weapons.

Gladiatorial combat was highly structured, with elaborate rules, not just a free for all.  But it had, as its common end, the death of a combatant.  If the body was still twitching, it was dispatched by a hammer blow by an arena employee dressed as an Etruscan god.

There is no comparison between gladiatorial combat and even the most violent sport of modern times, or jousting for that matter.  Take a look at the skull with the trident holes in it and imaging going to see that as standard entertainment. Roman civilization looks superficially like ours, but was deeply different.

If Romans had had realistic simulations of violent death, as we do, would they have needed gladiatorial combats?  Why did they "need" them in the first place?  Other cultures at the time, while having public executions of criminals, mass slaughters of fallen cities, etc., did not have such an elaborate practice of violent death.

I once wrote a story about a couple of animal trainers (violent, bizarre, and even sexual animal encounters were another big part of the show) who go off in search of a rumored hippogriff to kill in a show but instead find...well, now that I'm thinking about it, perhaps I should pull it out again.