Just a question, if only God can judge, for those of faith, isn't the Death Penalty usurping God?
God is not the only judge in that He is a God of order and has appointed rulers, judges, those who hold office in the lands. So there are judges in the land which do their duty and judge matters every single day. The problem is that not all judges are righteous judges. Many are crooked and for a bribe will look the other way or favor the wicked because they themselves are wicked. They weigh with false scales and God will judge them one day for it.
With that said, Murder requires the death penalty because it is the shedding of innocent blood and defiles the land. The judgment to cleanse the land of such defilement is the death penalty. This is God's Word. On the matter of a murderer who is on death row and becomes a born again Christian. They are forgiven their sins but still the consequences of that sin remain - that debt is due and the price of murdering an innocent person is death according to the Word of God.
Again, if we consider the shedding of blood - even done legally - in America - of 50 million babies - the land is defiled because there has of yet been no judgment. So when God's judgment falls you shall see that God does indeed take the shedding of innocent blood very seriously. The death penalty for the taking of an innocent life is a righteous judgment. It should stand.