Usually it was up to the audience, although high-ranking officials who attended the games could make the final decision.
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DIE
After losing, the crowd would show their opinion of whether or not they wanted him to die by turning their thumbs up or down. Ultimately, it was up to whoever hosted the games to decide if the gladiator lived or died. Usually, he would do whatever would please the crowd.
IT HELPED YOU DIE
they can get stabbed in combat and die from their wounds, or if they did not put up a good fight and lost, their opponent could get permission to kill them. Some of them were able to retire and died of natural causes.
According to Wikipedia: The now famous gladiatorial salute "Ave Caesar, morituri te salutant" or "Hail Caesar, they who are about to die salute you" is another product of movies. This salute was only mentioned by Suetonius (Lives of the Twelve Caesars, Claudius, XXI, 12­14) as happening once, spoken by condemned men (damnati) to Claudius at a naumachia (a staged naval battle) and they used the word "imperator" (Emperor) not Caesar.