So I looked back on chapter 2, but the only time the princess seemed empathetic toward the slave was before he abandoned his dignity. After he did, she turned into the sadist we know her as today. I'm sticking to my position that she is indeed a sadist. It's not some facade, or some way of cloaking supposed romantic feelings. She used to respect him, then, when he abandoned his pride, she lost that respect for him, and instead enjoys his suffering.
Stop giving her the benefit of the doubt. Just because she doesn't treat other people the same way she treats this slave, doesn't mean she's secretly a good person, pretending to be evil for some as-of-yet unknown reason. Is it really that hard to believe some royal would treat a slave like how most people would in a setting like this?
Regarding how Vera's supposedly too inconsistent, I can't agree at all. Why would he risk his life after having already abandoned his pride previously? What about the promise he made to his friends? Of course, the princess could be lying, but Vera's illiterate, has no contacts (other than his equally ignorant slave friends), and we can assume he has no education, thus no knowledge of the world around him. He has no means of knowing how likely it is that he signed a death contract or not.
I very much like the princess and am curious to see where the relationship between her and the slave will go. Will she start to empathise with him, or will he find out a way to outsmart her once?