Enough people have already quoted the verses which state that capital punishment is scriptural so I'm not going to repeat those. Some still want to say that even though the Bible is clear that the government should punish evil doers and that we have God's law stating how we should handle certain crimes, that we shouldn't condone it. Please take into context that the Bible doesn't say we as Christians and/or common citizens are to take the law into our own hands and kill anyone, not even in punishment of crime. That is the reason God ordained governments. We can pray and witness to the most evil of people and God can save them, but that does not negate the task that God gave to the government. God said in Proverbs 21:1, "The king's heart is in the hand of the LORD, as the rivers of water: he turneth it whithersoever he will." There are methods of petitioning for clemency and if it is God's will for a murderer to go free then God can turn the heart of the governor to grant such a pettition.
For those who point out that there are innocent people on death row. This is partly true, but also would not be a problem if the government handled the death penalty scripturally. Under God's law the death penalty was only used if there were at least two (preferably three) eyewitnesses to a crime.
Hebrews 10:28, "He that despised Moses' law died without mercy under two or three witnesses." With two eye-witnesses, you greatly reduce the instance of a falsely accused individual being convicted. As in the case with Jesus' trial they had trouble finding any two false witnesses who could keep their stories straight enough to get a conviction.
This raises an interesting question. Since the government is not doing things scripturally does that mean that there are "innocent" people on death row. Scripturally, the wages of sin is death and all have sinned, as such there is no such thing as an innocent. God can and has granted clemency to real murderers and we have seen the falsely accused go free as well. God can also use the government to punish someone for their sin that the government might not even be aware of. Psalms 17:13, "Arise, O LORD, disappoint him, cast him down: deliver my soul from the wicked, which is thy sword." God is in control and can use the wicked as his tool to bring about his intents. We are under a secular government with secular laws but as Romans 13 clearly teaches that God has ordained governments, even evil ones, for a purpose. Evidenced by the fact that Paul wrote that during a time that the rulers were known to claim to be gods.
For those who say lock them up and throw away the key I have a question. Where is the scripture for this position? We have many scriptures ordaining government implemented capital punishment which can be hashed and re-hashed over and over again. I've read through God's laws and the rest of the Bible and I have not found one single instance of God ordaining prison to punish a crime. There are examples of heathen nations imprisoning people, even God's people. There are a few instances of prophets being thrown in a dungeon, imprisoned in a home, etc, but that was during times of Israel becoming as much or more evil than their heathen neighbors and cared little, if at all, for God's law. None of the crimes mentioned in God's law (mosaic law) have prison sentences. The closest thing I could find to such is when someone had a potentially contagious disease they were quarantined until it had gone away or could be confirmed as non-contagious. Instead of prison, God's law always required restitution.
If I were stolen from today under man's law and if the criminal is caught and has money I might get re-payed but more than likely he simply goes to jail and I'm still without my money or stolen property. Under God's law if you stole you re-payed. If you had no money you worked until you could repay. A far more just system than man's law today. Under man's law today people commit crimes right in front of the police and plead guilty in court because they want to go to jail. They figure they are better off in jail than free. People in prison live much better than a lot of people walking free. They have their cable/sattelite tv, 3 meals a day, a library of books, a gym, etc, and all payed for by the free people outside who work for an honest living. Personally I like God's law better. It's far more just than what we are using today. As a Christian I have the option of forgiving a thief and him not having to repay me or do any prison time. So again God's law is found to not only be just but allows for mercy. Man's law is in many ways unjust and unfair, and only sometimes allows for mercy. Here in Kentucky we have what is called a commonwealth. That means even if I don't press charges the state can still prosecute on my behalf, which takes away my option of mercy.
If anyone can show me where God ordained prison in the his law then I would love to have that scripture. Until then I leave you with this verse:
Because sentence against an evil work is not executed speedily, therefore the heart of the sons of men is fully set in them to do evil.
Ecclesiastes 8:11