What does the Bible say about the death penalty / capital punishment?
Answer: The Old Testament law commanded the death penalty for various acts: murder (Exodus 21:12), kidnapping (Exodus 21:16), bestiality (Exodus 22:19), adultery (Leviticus 20:10), homosexuality (Leviticus 20:13), being a false prophet (Deuteronomy 13:5), prostitution and rape (Deuteronomy 22:24), and several other crimes. However, God often showed mercy when the death penalty was due. David committed adultery and murder, yet God did not demand his life be taken (2 Samuel 11:1-5, 14-17; 2 Samuel 12:13). Ultimately, ever since sin entered the world through Adam's and Eve's transgression and Original Sin, the punishment for sin has been death: “Therefore, as by one man sin entered into the world, and death by sin; and so death passed upon all men, for that all have sinned” (Romans 5:12). In reality, every grave sin we commit deserve the death penalty because the wages of sin is death (Romans 6:23). Thankfully, God demonstrates His love for us in not condemning us as we deserve (Romans 5:8) provided we repent and do penance for our sins (Luke 13:5).
When the Pharisees brought a woman who was caught in the act of adultery to Jesus and asked Him if she should be stoned, Jesus replied, “If any one of you is without sin, let him be the first to throw a stone at her” (John 8:7). This should not be used to indicate that Jesus rejected capital punishment in all instances. Jesus was simply exposing the hypocrisy of the Pharisees. The Pharisees wanted to trick Jesus into breaking the Old Testament law; they did not truly care about the woman being stoned (where was the man who was caught in adultery?) God is the One who instituted capital punishment: “Whoever sheds man's blood, by man his blood shall be shed, for in the image of God He made man” (Genesis 9:6). Jesus would support capital punishment in some instances. Jesus also demonstrated grace when capital punishment was due (John 8:1-11). The apostle Paul definitely recognized the power of the government to institute capital punishment where appropriate (Romans 13:1-7).
How should a Christian view the death penalty? First, we must remember that (1) God has instituted capital punishment in His Word, and (2) the Church has approved of this practice; therefore, it would be presumptuous of us to think that we could institute a higher standard. God has the highest standard of any being; He is perfect. This standard applies not only to us but to Himself. Therefore, He loves to an infinite degree, and He has mercy to an infinite degree. We also see that He has wrath to an infinite degree, and it is all maintained in a perfect balance.
Second, we must recognize that God has given the Church and government the authority to determine when capital punishment is due (Genesis 9:6; Romans 13:1-7). It is unbiblical and a heresy to claim that God opposes the death penalty in all instances or that the Church disapproves of it. Christians should never rejoice or be happy about that someone is killed when the death penalty is employed but should rather acknowledge that justice is being done; but at the same time, Christians must not fight against the government’s right to execute the perpetrators of the most evil of crimes.
Catholic view of capital punishment
The Catholic Church has always supported the legitimacy of the death penalty for extremely grave crimes. Canon law has always forbidden clerics to shed human blood and therefore capital punishment has always been the work of the officials of the State and not of the Church. Even in the case of heresy, the functions of ecclesiastics were restricted invariably to ascertaining the fact of heresy. The punishment, whether capital or other, was both prescribed and inflicted by civil government. The infliction of capital punishment is not contrary to the teaching of the Catholic Church, and the power of the State to visit upon culprits the penalty of death derives much authority from revelation and from the writings of theologians. The advisability of exercising that power is, of course, an affair to be determined upon other and various considerations.
St. Thomas Aquinas, Summa Theologica, Second Part of the Second Part, Q. 11, Art. 3: “I answer that, With regard to heretics... they deserve not only to be separated from the Church by excommunication, but also to be severed from the world by death. For it is a much graver matter to corrupt the faith which quickens the soul, than to forge money, which supports temporal life. Wherefore if forgers of money and other evil-doers are forthwith condemned to death by the secular authority, much more reason is there for heretics, as soon as they are convicted of heresy, to be not only excommunicated but even put to death.”