The truth however is, as always, a lot simpler than that. The Bible is very clear on what parts of the Bible need to be adhered to and it is really a pity an atheist who has read the Bible needs to point this out. So for my Christian readers here is your justification to kill witches, have slaves and not eat shellfish anymore.
The Bible has the following two verses which do not need much brain power to interpret.
John 14:15 "If you love me, you will keep my commandments."
This is Jesus speaking, and as Jesus is God that means the rules set by God/Jesus should be kept. So what did God want?
Psalm 119: 30 "I have chosen the way of faithfulness; I set your rules before me."
Simply put God wants you to obey his rules laid out in the Bible.
To sum up, this means the rules in the Bible are those you should live by. That is all there is to it. So now when you kill gays or witches, disrespect your women, eat shellfish or own slaves, just remember you are allowed to according to the Bible. However, also realize that the rest of the civilized world will not accept your stone age ideas and you will probably end up dead.
Harsh realities are not always the ones you may like.