Yes, we have killed innocent people. But then, innocent people have also died and suffered because we haven't killed the guilty ones. The one advantage of the death penalty for the guilty is that they never re-offend.
My guess is that innocent sufferers far outweigh innocent death sentences.
So I conclude that until we have a legal system, and more importantly, a judiciary prepared to implement it, which puts these people away in a tiny cell with nothing more than cheap food at a total cost of less than $1,000 p.a. - we need the death penalty to protect the innocent.
