I don't really consider any form of non consensual (or consensual in the form of a contract you can't leave early) ownership of another human to be "worse" than any other form. This includes indentured servitude.
I think we're talking about different things, I don't consider the concept of ownership to be worse, I do consider the treatment of slaves to differ. I made this comment before I saw your clarification.
2) but ultimately I don't know where you even begin to measure whats worse in terms of treatment. I presumably varied wildly master to master, throughout all history and all countries. Vedius Pollio for example used to throw his slaves into a pool of eels to be eaten alive.