Most gladiatorial events weren’t fatal. That’s a Hollyweird invention. Making them fatal would be like entering your Ferrari in a demolition derby. Most of the events were racing and animal fights - animal vs animal, man vs animal, etc. This is something that still goes on - bullfighting,
dog fighting, etc. Immoral by our modern standards but once very common. Public executions there did occur & Christians to the lions was very rare again in spite of Hollyweird and religious traditions.
Public executions in the Colosseum were rather common but were mostly* reserved for convicted criminals (murders, thieves, rapists, kidnappers, runaway slaves, etc.) and those captured in war who were not deemed good for being a slave.
The method of execution varied from crucifixion, being burned alive, to being thrown to wild beasts to being tied to “ships” which were sunk in mock navel battles leaving them to drown or a beheading which was mostly reserved for higher status Roman citizens. One of the more interesting forms of execution was catapulting a condemned criminal across the Colosseum in a re-enactment of Icardis flying too close to the Sun. It never ended well for Icaris.
The executions of Christians, starting with Nero who scapegoated them for the great fire, waxed and waned over time.
Emperor Hadrian (117–138) for instance, responding to a request for advice from a provincial governor about how to deal with Christians, granted Christians more leniency. Hadrian stated that merely being a Christian was not enough for action against them to be taken, they must also have committed some illegal act. In addition, “slanderous attacks” against Christians were not to be tolerated. This implied that anyone who brought an action against Christians but whose action failed, would themselves face punishment.
The persecutions of Christians started back up again during the reign of Marcus Aurelius and his son Commodus, who by the way did fight as a gladiator, probably in fights rigged in his favor but he was not killed by Russell Crowe in the Colosseum. 😊
It is common misconception that Christians were persecuted solely for being Christians, well sort of, they were from time to time but not universally or all across the Empire and up until the time Constantine made Christianity the official religion of the Empire.
The Romans mostly didn’t necessary care what God or Gods one worshipped in private. There were many “cults” who worshipped foreign deities including ISIS (no, not that ISIS) and there is evidence that Roman troops in England took up worshipping the local Celtic deities (when in England, do as the English do.) LOL!
But what they did object to was what we might call Blasphemy or Treason to the Empire. That meant they expected all citizens to make sacrifices, even if just symbolically to the Roman Gods and to the emperor.
Christians who refused to recant by performing ceremonies to honor the gods were severely penalized. Those who were Roman citizens were typically, but not always, exiled or condemned to a swift death by beheading while slaves, foreign-born residents, and lower classes were more likely to be put to death by wild beasts as a public spectacle.
While perhaps not a good analogy, it would be as if one here in the US could be arrested, tried and executed for not reciting the Pledge of Allegiance or for taking a knee during the National Anthem or not reciting the Lord’s Prayer during a public meeting or in a school.
And yes, not all gladiators were killed in the arena. You are correct. The investment and training of a gladiator was a significant investment, and no one would put in that sort of investment if there was a 50/50 chance their investment would be killed each time they fought. There has also been evidence that gladiators were given medical treatment (such as it was back then) and fed very well. Some of the top gladiators were seen like rock stars are today. Think of boxers and Mixed Martial Arts fighters in our world today.
That’s not to say that gladiatorial combat didn’t come with a risk of death but those who survived were often granted freedom and Roman citizenship. And not all were slaves. Some free Roman citizens, albeit of the lower classes, volunteered to become gladiators as it could be very lucrative.