Christianity and the death penalty

This article takes a more religious approach than usual, but this is mostly for Christians who support the death penalty.
Lots of pro-life Christians say that it's hypocritical to be Christian and pro-choice. I agree (of course, I think that's true for anyone who is morally against killing people), but whenever I hear that, I think of how Christians also have different views on the death penalty, and how that's considered "okay."
Cardinal Joseph Ratzinger (later Pope Benedict XVI) wrote this in 2004 regarding this issue: "While the Church exhorts civil authorities to seek peace, not war, and to exercise discretion and mercy in imposing punishment on criminals, it may still be permissible to take up arms to repel an aggressor or to have recourse to capital punishment. There may be a legitimate diversity of opinion even among Catholics about waging war and applying the death penalty, but not however with regard to abortion and euthanasia." This doesn't make any sense to me. Why is it okay for Christians to support capital punishment, but it's not okay to support abortion or euthanasia?
A common argument is that victims of abortion and euthanasia are innocent. I'm sorry, but only giving innocent people the right to life doesn't sound very "Christian" to me. Jesus taught that we should love our enemies and be merciful. Luke 6:33 also says, "And if ye do good to them which do good to you, what thank have ye? for sinners also do even the same." The Message Bible says it this way (which I personally think sounds better): “If you only love the lovable, do you expect a pat on the back? Run-of-the-mill sinners do that."
Now, I'm not a die-hard pacifist, and I'm all for defense of self and others. Nor do I think anyone should feel like they have to bake a cake for Hitler (or any other evil person). That is not how I interpret Jesus' teachings. But capital punishment clearly goes against Jesus' teachings. Of course, there also secular reasons to be against it. The death penalty is premeditated and not necessary to save a life "in the moment." Not only are there better alternatives to capital punishment, but from what I know, it may be ineffective.
If you still only think innocent people deserve the right to life, consider these wise words from Gandalf (even if you're like me and you've never read Lord of the Rings): "Many that live deserve death. And some that die deserve life. Can you give it to them? Then do not be too eager to deal out death in judgment.  For even the very wise cannot see all ends." Everyone deserves a second chance (which is also a message found in the Bible), and their punishment should not make them unable to fix their mistakes. Capital punishment doesn't teach the criminal anything, nor does it give them a second chance. I strongly believe that people can change, and that God is merciful. No one can undo murder, but technically, no one can really "undo" any other sin.
Lastly, and perhaps most importantly, innocence does not determine personhood. It determines how moral they are, but it doesn't determine their personhood, or whether or not they're children of God.