Genesis and capital punishment

The OneYearBibleBlog reading for January 4^th included Genesis 9:6

Whoever sheds the blood of a human, by a human shall that person's blood be shed; for in his own image God made humankind. (NRSV)

At first blush, this would seem to support the idea of capital punishment, and I've seen discussions about this passage that say exactly that. But I think this to be a false reading. I don't believe this passage to be an instruction by God that murders must be killed. Rather, I think this is a warning by God of natural consequences. Notice that, while the verse says that "... by a human shall that person's blood be shed ..." it does not excuse the second killing from being under the exact same judgement!

In light of that, I think this passage is actually an indictment against capital punishment specifically, and violence in general, for any reason. Even legally "sanctioned" violence, Just because God declares that those who kill will themselves be killed (those who live by the sword die by the sword) does not mean that we, in the form of our government, have been elected by God to carry out that judgement.