I believe you left out the last part of that principle.
"An eye for an eye, and the world goes blind."
That's a prettyy good aphorism to think about with this situation. I think it would be much more of a punishment to have to rot in jail for the rest of my life then it would be to get a free pass out of life in prison. Also, it's a given fact that it costs more to execute someone then it does to have them live in prison (at least, it is in the US). I don't think we have any right to take the existence of someone into our own hands, regardless of what they've done. Also, (again, speaking about US) the forms of death penalty used are lethal injection (which does cause pain), firing squad (obvious pain), and the electric chair (which regardless of pain is expensive). You cannot tell me that you would like to be subjected to that kind of torture.
Okayyy, that one was a little too "religious" for me. I'm a firm believer of separation of church and state. The bible also said that giants walked the earth and Lot's wife was turned into a pillar of salt. Obviously the people writing the thing didn't fully understand science and nature. Besides, the Epic of Gilgamesh or the Odyssey hold about as much credibility as the bible, in terms of preciseness. I mean, it was written to be symbolic and hold religious truth, not scientific fact. You can't/shouldn't take it literally. But I digress...
Anyway, I don't think the bible should have any say in whether the death penalty should be allowed.