This is the flip side of the argument that innocent people will be mistakenly executed when the death penalty is legalized. While that may be true, the number of innocent people mistakenly executed will probably be much lower than the number of innocent people murdered by convicted murderers who are not executed.
I believe the number of innocent guys executed would be practically nil, in part because there are so few executions anyway. Plus most executions only happen after years of constant appeals, re-trials, and examining of evidence. The scumbags have so many advocates groups working to keep them off death row. If there were as much work going on to keep them behind bars then I think fewer people in society would be victimized.