I'm skeptical of your claim that "most" gladiators were just professionals freely doing a job. If so, please cite these "old actual sources".
As to your claim that modern combat sports aren't safe because some athletes have suffered injury, I'd like to hear why that doesn't apply to any sport. Or even any job where someone at some point gets injured. The fact that a vanishlingly small number of MMA contestants have "spoiled their future" due to injury is irrelevant without knowing the broader context of how many haven't, how it compares to other activities, etc.
Lots of activities are risky, that doesn't make them morally reprehensible and it's baffling to me that anyone can make that claim with a straight face.
Even if it WAS very dangerous and they freely choose to compete in a sport where half will end up with a lifelong injury, it still doesn't have anything to do with slavery, animal torture, murdering civilians, etc.
#1 talks about how free vs slave gladiators changing over time.
Most interesting to me was #3, where it talks about how most were not killed but wounded instead. "historians have estimated that somewhere between one in five or one in 10 bouts left one of its participants dead."
So were most gladiators, it was a an actual job for many of them, not everything was like on Hollywood movies.
People should read more old actual sources of information and watch less movies.
As for being safe, tell that to the ones that have spoiled their future thanks to major injures to member or head concussions.