@macarroiz You are wrong about that. In ancient times, elite slaves (personal bodyguards, teacher, butler, ...) had quite a good status. So if you were a slave coming from the slums or from a war (village), being trained into becoming an elite slave was a salvation, because you would never have a status as high if you were free. But it's true that it was extremely rare, most elite slaves had a higher status when they were free (nobility, soldiers, ...).
In this story, we can suppose he was from the slums, so him being trained as an elite slave is in fact a good thing.
But I agree that slavery was never a good thing globally, that it was something that needs to be abolished. If all slaves were of the elite category (who were an investment), it could be a good thing; but it always was low-level slaves in majority (not treated better than a beast, cheap man power, just a merchandise). It's even worst now since nearly all slaves are of the low-level category.
Btw, they were 3 different categories: low-level, elite and gladiator (only for the roman system)