So, you would paralyze valid societal actions for fear of making a rare error? Even if it ends up costing more innocents their life due to your inaction? If Clarence Ray Allen had been executed for the first murder he committed, three more innocents would be alive today.
Juries are not infallible. Especially with DNA advancements.
And if the DNA proof is conclusive? If the killer confesses? If the murder was done on live TV, by an assassin?
Keep them locked up. Look around, the death penalty certainly hasn't stopped a single murder that I know of. Put yourself in the place of the poor smuck that goes to the gas chamber for nothing. How do you know how rare it is?