A lot of people have a hard time understanding the severe punishments required by the Old Testament law. Isn’t it true that Christ redeemed us from the curse and not from the law? The law of the Lord is perfect and this perfection is what revealed our curse. The law was our tutor who reared us up that we might become mature. Why would you go back to your teacher when you are fifty years old, unless to say thank you? The law made us mature, so that we could grow up to be prepared for the coming of Jesus Christ.
When we study the law, we simply are studying the curse it reveals in us. The law put us under a curse, only so that Christ could set us free from it. It is a grace that the curse was revealed by the law, because the law is what made us aware of what the work of Christ was meant to be. This was true for the entire world, because it was the entire world that was under a curse. This was the curse of death—and the law revealed that we deserve the punishment of death for our sin. This is why the sins of the people required a retribution in blood, whether it was their own by execution or an animals through the sacrificial system. These punishments were the revelation of our curse—and the Jews under this curse were saved from it just as much as we are now. The religious experience of the ancient Israelites is the same religious experience as a modern day Christian. It is the same hope, the same faith, the same salvation, and the same broken and contrite spirit that God desires.
Capital punishment was a severity that all men deserve, but no one in Christ ever had to receive. A tower does not fall on someone because they are unrighteous, but a tower will fall on anyone who is unrighteous. The requirement of capital punishment is part of the curse that we are now set free from. This is why those in Christ rise from the dead: it is the breaking of the curse.
It seems to me that the biggest question is not why the Old Testament law was so severe, but rather why God did not give mankind the law sooner, that those generations after Adam and Eve might have experienced the grace and clarity of knowing what they were saved from—and how they were to be saved.