Why do I prefer our United States Constitution to the Bible? Lots of reasons, but I'll focus on one. The Constitution allows for do-overs. Its authors understood the document to be imperfect and made provisions for future generations to amend it.
Alas, there is no such biblical escape clause. What you see from way back then is what you get.
Neither the Constitution nor the Bible included freedom of religion, equal rights for women, prohibition of cruel and unusual punishment, or the abolition of slavery - but today, through amendments, the Constitution does. We also have a democratic form of government that allows for progressive laws that our 18th-century founders might not have considered or desired.
So what about those who believe the Bible to be the inspired word of God, yet can't reconcile some portions with a loving deity? It's difficult to justify passages about killing witches, slaying all women and little children in a city, the blood of Jesus being on all Jews and their children, killing homosexuals, and many more. Even biblical literalists now try to interpret some of these passages in more enlightened ways.
Not only is slavery nowhere condemned in the Bible, but some have used Noah's curse of Canaan to justify it. ("Cursed be Canaan [presumed black]! The lowest of slaves will he be to his [presumed white] brothers.") Since nobody today condones slavery (and groups like the Southern Baptist Convention have even apologized for promoting slavery), interpretations abound. For instance, Ken Ham of Answers in Genesis says Noah's curse had to do with a rebellious son, not skin color.
Apparently, it's easier for some Christians to be on the moral rather than on the scientific side of history. Albert Mohler, who Time called the "reigning intellectual of the evangelical movement in the U. S., believes that the incredible Adam and Eve story goes to the heart of Christianity - because the whole point of the crucifixion and the resurrection was to undo Adam's original sin, and that without a historical Adam, the work of Christ makes no sense.