Becoming a gladiator was actually one of the lighter punishments in the Roman world, provided the punishment included training. It provided the criminal with a chance of living. Being sentenced to the mines, crucified or being thrown to wild beasts meant death. Later in the Roman era, there was also a special type of gladiatorial combat -- gladiatores meridani -- these were robbers, murders, people who had committed treason and arsonists whose crimes had earned them death in the arena. Christians were considered treasonous because they refused to recognise the Emperor as a god. Such people were not real gladiators, but faced wholesale slaughter -- unarmed combat against an armed foe with the prospect of the winner being disarmed and facing the next foe. They were the luncheon time spectacle in Rome.
True gladiators were trained and even though it was a hazardous occupation, it did provide an opportunity for wealth. If a man (or in far fewer cases a woman -- women were banned in 200 AD) was sentenced to be a gladiator with training -- the usual sentence was two years of training school and then three years in the arena. After which time, they were free to leave or to become contract gladiators. Because of the cost of training gladiators, as time went on, they used more and more untrained criminals, rather than proper gladiators. A properly trained gladiator stood a good chance of surviving, in part because of his value to the lanista or owner of the gladiatorial troop.
Basically, it was only in Rome where the bulk of the money was that trained gladiators actually battled to the death. Recent excavations of a gladiatorial cemetery in Asia Minor have confirmed that the vast majority died with a blow to the back of the head. In other words, they had suffered too severe injuries and were killed to end their suffering rather than being run through with a sword.
So who were trained gladiators? They were mainly prisoners of war and slaves. Because of the wealth that could be made, they also included a variety of social outcasts and desperate adventurers. These freemen became contract gladiators. Being a gladiator was considered to be worse than being an actor. One of the reasons for this was the oath all gladiators were forced to take. This allowed them to be burnt (tattooed), shackled with chains, whipped with rods and killed with steel. For Romans, to sport a tattoo was to show that a man had been a slave. It was not a mark of honour and precluded them from serving in the Army.
So hopefully, you can see from this piece, that while the Arena was used in punishing crime, not all gladiators were criminals.