This is floating around the internet.
The command is actually: you shall not murder. There is a big difference between a government lawfully killing a criminal, which the Bible permits, and murdering someone. Murder is the taking of a innocent life. Killing someone via the death penalty is fully justified because the person being killed has killed another, destroying an image bearer of God. This is why the LORD gave us the command in the first place. It is to deal with those who have sinned against God by destroying His image in another person.
Please do not be duped by those who misquote the passage from the Ten Commandments.
UPDATE: Another thought that occurred to me is that those who love to misquote the Sixth Commandment when it comes to the death penalty, are usually quite silent when it comes to using the same verse concerning abortion. In fact, the correct version of the Sixth Commandment applies directly to the issue of abortion, in that the killing of a child in the womb, does fit the definition of murder.