There are many arguments to be made against the death penalty. Nick Gillespie summarizes three in this article for Reason Magazine - it's expensive, it doesn't deter crime, and the government shouldn't kill people. The Death Penalty Information Center argues that death sentences are racially biased: not only are black men disproportionately sentenced to death, but darker-skinned black men are sentenced to death more often than lighter-skinned black men. The Innocence Project reminds us that innocent men are often sentenced to death - and, in Louisiana, over half of the men sentenced to death in the last thirty have had their sentences overturned. Even conservatives are concerned about the death penalty, citing several problems with executions in the United States.
If you're inclined to be persuaded by any of these arguments, that's fine. I'm only going to deal with one, the one that I think is most relevant to any libertarian discussion of the death penalty: The government shouldn't kill people.
Let's consider the powers of the state. We've discussed already that individuals own themselves, and that the primary (or only) purpose of the state is to protect the rights of its citizens. To this end, the state may use force in defense of the lives, liberty, and property of the people. These rights, however, do not belong to the state, for states have no rights; only individuals have rights. How, then, is the state to gain the power to protect the rights of the people - the power to govern?
The old argument was that the state was endowed with authority by God. This was known as the Divine Right of Kings. The world had a natural hierarchy; God ruled over all, kings ruled over men, men ruled over their wives. Only the king had rights. Later, this was modified; the Magna Carta ensured that nobles, too, would enjoy rights.
With the birth of liberalism came the radical notion that each individual was endowed with rights, not just the king and the nobility. Accompanying this idea was the concept of consent of the governed. The state had power, not because it was endowed with this power by God, but because it was endowed with this power by the people.
Each individual has, for example, the right to self-defense. If you attack me, I may defend myself; if you use lethal force against me, I may use lethal force to stop you. This right can be delegated. I can hire a bodyguard skilled in violence to protect me. Together with my community, we can hire police, authorized to use violence in defense of all of us.
However, I do not have the right to murder. If you attack me, and I prevent your attack, and then we meet several days later, I may demand recompense for my injuries. I cannot, however, shoot you in the head. I cannot tie you up and then leave you to die without food or water. I cannot lock you in a secure room and inject you with lethal drugs. That would be murder.
Because the authority of the state derives from the citizens, while the scale of the state's authority may exceed the scale of any individual citizen, the state does not gain any authority that the citizens do not possess.
The state (and its agents) may not walk up to people and shoot them in the head. It cannot tie people up and leave them to die without food or water. It cannot lock people in a secure room and inject them with lethal drugs. That would be murder.
The presence of due process does not make the murder any less of a murder. If a citizen does not have the right to gather likeminded citizens, vote to kill someone, and then kill them, then neither does the state, regardless of the forms and procedures followed. A mob of townsfolk lynching someone accused of a crime, and a solemn jury agreeing that the state's executioner should do it for them, are not fundamentally different.
The state cannot exercise any authority that is not given to it by the citizens, and the citizens cannot give to the state any authority that they themselves do not possess.
No citizen has the right to put another to death, except in defense against lethal force being used against them. A prisoner incarcerated for their crimes does not pose a lethal threat to the citizens, and so regardless of their past crimes, cannot be morally executed.
