Telling masters to be kind to their slaves is all very well, but far from representing moral progress, that just has the effect of reinforcing the institution of slavery by putting a kindly face on it. If slavery was so emphatically against the grain of the gospel, then why not just make it a requirement of church membership that one cannot own slaves? (p. 24)
The argument made by many theists that the Bible actually regulated slavery and made it more humane compared to other cultures still does nothing to denounce the institution itself. Trying to argue that regulating slavery somehow makes slavery morally acceptable would be like trying to argue that regulating how often and severe a pimp can beat his prostitute makes it morally acceptable. Theists are just kidding themselves, they really are. I think it is psychologically difficult for many theists to come to the acknowledgement that their god could be capable of the type of cruelty that the new atheists accuse him of.
Regarding the indentured servitude theory, was it really as humane as many Christians make it out to be? This is another popular tactic made by the apologists. Thom Stark addresses this in detail in his book. He writes:
It’s true that Hebrew male slaves served only a term of six years, to be released in the seventh, but this was emphatically not true of any and all non-Hebrew slaves, despite Copan’s attempts to force the text to say otherwise. Moreover, most ancient Near Eastern societies had release laws comparable to Israel’s mandates, and while a six year term of service was stipulated in the laws of Moses, only a three year term of service was permitted in the Code of Hammurabi! (p. 165)
I always knew that biblical slavery was indeed slavery. And even if it wasn't, indentured servitude where your "master" is allowed to beat you within certain regulations, I cannot imagine an all-knowing and all-loving god permitting or commanding. Rather, these biblical "morals" are much more obviously the product of a Bronze Age, Near-Eastern tribe full of superstition and xenophobia, and that's why they make little sense in today's light.
Another website of New Zealand Christian apologists called thinkingmatters.org expresses concern over whether the OT god changed his mind. According to the blog, god in a way compromises and gives moral commandments in the OT that are an improvement over what existed before, but are still less than ideal. If that's true, god's moral commandments in the OT are intentionally imperfect, but function as a "compromise between the ideal and the enforceable." It's funny that this same blog praises Paul Copan's responses to the criticism of the new atheists apparently unaware of the his on-again off-again relationship to the truth.
If misguiding your readers and compromising on the truth are what you need to sugarcoat the god of the OT so he doesn't look like a moral monster, then you're really just following in the footsteps and examples set by the god of the Bible.