Usually when I read books in ancient-like settings (settings that are either in real ancient civilizations on Earth, or fantasy settings similar to those), there are relatable characters who employ modern humanitarian ethics such as caring about the welfare of criminals or war prisoners from a different state, or wanting equal treatment for people of different sexes and ethnicities.
However, when I read historical accounts of just about any civilization prior to the 18th century, I get the impression that ethics like these were virtually nonexistent. People had circles of empathy, but the idea of, say, caring about people in different states, especially war prisoners, wasn't even really considered. Even for people in their own states, most people didn't balk at the idea of impaling someone in a public square for trivial crimes like stealing or speaking out against their monarch. Slavery was practiced in just about every civilization that had the means to do so, prior to the 19th century. Sexual assault of the worst kind was seldom considered an offense against the woman, but at worst, it was considered an offense against the man who "owns" the woman (the husband, or father if the woman is unmarried). There are plenty more examples of values that would make our modern stomachs turn, but were considered perfectly acceptable, and even commendable.
I'm not a historian. The bulk of my understanding of ethics in ancient civilizations comes from reading small articles and popular books (The Better Angels of our Nature is what gave me the most recent impression of historical ethics). So maybe I'm wrong about this. But I generally get the impression that, in ancient-like settings in fiction, characters (usually protagonists) are given ethics that are far too modern to reflect even the best people in actual ancient civilizations.
Suppose I want to write fiction in an ancient-like setting. While I'm not going for totally unambiguous heroes and villains, I do want to have characters for whom the reader will have varying levels of sympathy. I want the readers to be able to follow some characters and hope they succeed. However, I also want people's ethics to generally reflect real historical civilizations that were as close as possible to the fictional setting in which I'm writing. So I want even the most sympathetic characters to be perfectly fine with certain contemporary values that we would generally consider abhorrent. For example, if the setting was similar to the Roman Empire, main characters should be okay with crucifying people in public squares for stealing, owning slaves and pitting them in deadly combat for entertainment, slaughtering regular citizens in a foreign town during a war/raid and letting your soldiers enjoy their "spoils", etc. But the "good" characters would still generally care about their fellow (free, usually male and property-owning) Roman citizens, have codes of conduct for things like honoring a deal or contract, have integrity and stay true to their word, express humility when warranted, love and make sacrifices for their family, etc.
I'm skeptical that even I could possibly sympathize with any character in an ancient-like setting with ethics that realistically portray that setting. Would the readers respect varying levels of moral values that would normally differentiate people between "good" and "bad" in those settings, if, by our modern standards, basically everyone is a monster? Is it possible to do this without alienating most of my readers? Or am I forced to suspend some disbelief and impose unrealistically modern ethics to people in ancient-like settings just to make the story compelling enough to follow?