Atheist vs. OT
It usually goes like this:
Atheist: The God of the OT is evil.
Theist: How so?
A: He kills babies, encourages slavery, and punishes people for not worshipping him.
T: Ok. He kills babies. So?
A: You don't see a problem with that?!
T: It depends on the situation. All babies have a fallen nature and are under God's righteous condemnation. We all deserve death, according to God's law.
A: Babies deserve death? They haven't done anything.
T: It doesn't matter. It is in their nature to break God's law. God has a lawful right to kill any baby that He wants.
A: That's sick.
T: Just because you don't like it does not mean it is wrong. God is eternally holy and righteous. When we sin, it is eternally against Him and warrants an eternal punishment. To have a sinful nature is to be inherently against Him. But God is also merciful, and he may have very well allowed the babies into Heaven, which would mean that he did not punish them at all. Neither you nor I know either way.
A: God allows slavery.
A: Slavery is wrong.
A: It devalues people.
T: I'm glad that you concede people have value. But okay, God allows slavery. Do you have a point?
A: God can't be good if he allows slavery.
T: Just because he allowed it does not mean he liked it.
A: Then why did he say that it was okay to beat a slave?
T: In biblical times, slaves were people who offered themselves as personal property in order to pay a debt. They entered into what amounted to a contact, giving masters the right to use physical punishment if need be. God in no way encourages it, he simply allows it but warns masters that if a slave should be beaten to death, then the slavemaster will be condemned. Likewise, God made it clear that no human has a right to own another human without consent.
A: Why didn't God just tell us to abolish slavery?
T: There are many things God could have told us that he did not tell us. He was operating within a particular time period and dealt with people in the context of their present social norms. God's revelation wasn't about giving people easy solutions for all of eternity. He wanted his message absorbed in a particular way at a particular time. In 1,000 years, people may look back at us and say that we were barbaric and God should have told us to change.