Answer:
The audience justified this because the vast majority of gladiators were slaves, and slaves, in Ancient Roman society, were not considered people worthy of civil, political, or personal rights, they were essentially property.
These slaves were mostly taken from the frontiers of the Roman Empire, where constant warfare generated a steady supply of slaves. The strongest were to become gladiators, while others were to be worked in farms, or in domestic households.
In general terms, Ancient Roman society largely depended on slavery to work property, it was considered to be a slave society.