John Shook makes a case against the death penalty on humanist grounds, at the Center for Inquiry website. It's a short, compelling piece, and I recommend reading it. Here are some quotes and commentary.
Dominated by that vengeful spirit, the criminal justice system encourages prosecutors to chase a conviction of whoever they can, rather than the truly guilty; it distracts jurors from the lofty standard of reasonable doubt; and it lets supervisory courts forget their supreme duty of justice for all. In that heated atmosphere of swift vengeance, the criminal “justice” system mostly executes the poor, the disadvantaged, and racial minorities.
The pragmatic argument: even if you agree that killing people is okay, the system we have doesn't work. We kill the innocent, the poor, and the marginalized.
Pro-deathers should broaden their principles. Governments exist not merely to deliver criminal justice, but to protect and defend the lives and rights of everyone. When a government executes an innocent person, it violates the ultimate justification for its own existence.
Right on. The point of government is to protect its people, not kill them. I realize this makes some people uncomfortable, but criminals remain citizens. If we want to abdicate responsibility for the people we imprison, the answer is exile. I'm sure there are countries out there that will take them. But if we're going to keep them here, we do have obligations to treat them like human beings, and that means not killing them.
And finally, the argument in a nutshell:
Humanism stands for valuing the lives of all, individual human rights, justice for everyone, and governments that defend all of their people. These grounds alone are sufficient for abolishing the death penalty.
via Friendly Atheist