The Islamic View

Do you not love to be forgiven by Allah?

Whoever will come to Allah with an evil deed, shall be requited with no more than the like of it. They shall not be wronged.

Take not life, which God has made sacred, except by the way of justice

Introduction
Islam is an Abrahamic religion, based around the words of the Quran, the holy book, and the religions founder, the Prophet Mohammed. There are around 1.9 billion Muslims worldwide, ranking it the second largest religion in the world, and it has monotheistic beliefs, worshiping Allah/God. In Islam, the views about punishment are centred around God's punishment, the day of judgment, the rule not to kill and also the idea of forgiveness.
Forgiveness
The idea of forgiveness is one of the key beliefs encouraged in Islam, the Quran states, "whoever is patient and forgives – indeed, that is of the matters [worthy] of resolve" and "Do you not love to be forgiven by Allah? And Allah is All-Forgiving, Most Merciful" meaning that this is the highest valued quality of a Muslim, and the second quote fits in with the concept that often the followers of religions aim to reflect the same values that their God has, which in Abrahamic religions, includes all-forgivingness. It is recorded that the Prophet Mohammed said "Be merciful to others and you will receive mercy. Forgive others and Allah will forgive you" which would lead Muslims to want to forgive those who have wronged in exchange with mercy for any immoral acts they have performed. In the situation of the criminal being sentenced the death penalty, a Muslim would chose to be merciful towards the criminal because there is an aim to reflect that value of Allah and because they would want mercy for themselves and deeds.
God's Punishment and The Day of Judgment
Another central idea in Islam is Allah's punishment and how it is always fair, demonstrated in the quote "Whoever will come to Allah with an evil deed, shall be requited with no more than the like of it. They shall not be wronged" meaning that whatever punishment one receives was deserved because it is an equal punishment to the crime committed. This links to the day of judgment because any punishment that is deserved will happen then, meaning there is no need for punishment on earth and forgiveness should be practiced. A muslim would believe that whatever punishment the criminal deserves whether that be death or another punishment, would be carried out by Allah and therefore the death penalty should not be implemented because that is taking the responsibility of Allah and creating punishments for the criminal, especially when the criminal could have been wrongly convicted and therefore wrongly punished.
Is the Death Penalty Ethical?
The Quran also says that "And do not kill any one" which clearly prohibits the action of the death penalty. However this is situational because in 6:151 it says "take not life, which God has made sacred except by the way of justice" which proves that on a whole Islam agrees with the death penalty to be used for when someone has committed homicide. On the other hand, more lenient punishments are considered because there is a belief that there will be a more fitting and deserving punishment at the hands of God. Overall Islam agrees that the death penalty is ethical and allowed because although there will be a more profound punishment from God, there is still room for a worldly punishment, whether that be the death penalty or not.