In the US slaves were used as collateral for loans. In a way slaves weren't owned by plantation owners, they were owned by the banks. That if anything explains some of the pernicious aspects of US slavery. Also likely the US banking system. And the mistreatment of blacks after the civil war.
But as far as I can tell, the Romans were similar in a lot of ways to the Spartans. Probably everyone was in the region, the Spartans were just a bit more extreme, and had the best propaganda.
But look at what the Romans had - pater familias (the father of a household could legally murder a weak infant, or technically anyone under them including any adult male sons), legends like Troy and Horatius Cocles, plantation slavery to keep the city fed, and a violent and hierarchical society run almost entirely by veteran officers from noble families - see https://en.wikipedia.org/wiki/Cursus_honorum
Just like in modern USA, it's nobody's business if I cut off one of my fingers, it would not be of concern for such a society if a family kills one of their members.
If you kill or steal from another family, legal systems usually step in.
I always keep i mind that through almost all history, compared to our modern life, people were incredibly poor, living near famine, disease, and death. In that light what seems barbaric is often just practical.
Honestly, I think you're the one who needs to be providing the evidence, especially if you're claiming anything close to the opposite of that (i.e. that most slaves in Roman times were freed).
Roman slaves were property and could be treated as such, including sexual exploitation (eg made to be prostitutes) and being summarily executed.
And yes, slaves could be freed but that was also true in the American slave era. Just because something is possible doesn't mean it's accessible, particularly by the slave themselves. Also, freedmen in Rome had less rights than citizens.
The history of Rome is a fascinating one but it's hard to look at through the eyes of modern morality. I think this generally applies to the Ancient World: if somehow we could be transported back to that time I'm sure we'd find it quite alien.
My favourite example of this is the origin of the word "decimate" [1]. If, say, a military unit rebelled against Rome, groups of 10 would have to decide which one of the 10 would die and to kill them.
Slavery has always been brutal.
[1]: https://en.wikipedia.org/wiki/Decimation_(punishment)