This from a comment I made in a blog discussion. It’s not a full treatment, just a few things to think about.

I think your concern is VERY common one. I hear it articulated honestly and sincerely.

1. As Christians the OT is modulated by Jesus as the final Word of God, the word becoming flesh. The reason Christians affirm the OT is because Jesus did so, but note how Jesus himself feels free to change it. Read the sermon on the mount and see how Jesus reinterprets Moses. How many times does Jesus seem to really upend some of the stuff in the OT, the woman caught in adultery. Closely read how NT authors interpret OT authors. All of this is happening while Jesus continues to also strongly affirm the OT. This should lead us to believe that there is more going on than the simplistic moralistic judgments we like to label things with. Peter Enns is good with some of this stuff in showing that the internal conversation of Scripture is far more nuanced than the simplistic promotional language the church too often broadcasts. You don’t have to lose authority to recognize the complexity of the issues.

2. I also hear concern that the OT will sanction all sorts of evil. Have people used the OT to sanction evil? Absolutely. Then again people do evil without any sanction at all or make up reasons to sanction evil with or without the Bible. We do evil. We also like to maintain our own sense of righteousness and when we do them both at once we’ll concoct nearly anything. Blaming the OT for the evil we do can be seen in some deep sense as yet another example of our own self-righteousness (and see where self-righteousness can take you).

3. Don’t read the OT as a flat document. Kids often view their parents this way. Kids love to point out inconsistencies in their parent’s behavior, and sometimes the kids are right, but sometimes the kids can’t understand a broad and complex situation like the parents can. Parents of multiple children soon learn that different kids need different rules. Does this break the fairness rule? Yes. Sometimes the fairness rule needs to be broken because of the rule of love.

4. The OT itself is not blind to its own issues. This library (it is more a library than a book really) has both the genocide of the Canaanites and the sparing of the Ninevites and attributes both to the same God. My favorite line in the book of Jonah is “and God repented of the evil he said he would do to them…” This of course makes you scratch your head if your doing systematic theology, but it’s right there in your KJV Jonah 3:10 because it’s there in the Hebrew. If you were patching together a propaganda piece for your big box evangelical church you’d want to fix this stuff, but there it is and there it will remain because the church at least on some level tries to hold itself accountable to something outside its own agenda. None of it is new.

The OT psalm writers and prophets had their way of dealing with these issues, they complained to God which demonstrated honesty and trust. We should do likewise.