We don't have laws to exact revenge for misdeeds. We have laws to protect the law-abiding. An eye for an eye just leads to a world full of blind people.

That said, I'm not really against the death penalty for the most heinous of offenses, in principle. I'm against the death penalty because it costs the taxpayers more to kill someone than to just lock them away forever. I don't think we should execute anyone at all though unless the evidence against them is absolutely ironclad, backed up by video and DNA preferably, because once you kill someone you can't take it back if it was an error, and 'oops, sorry, it was just an innocent mistake' doesnt cut it as an excuse.