Consider this: As a proponent of the death penalty (both as punishment and, to a lesser extent, as a deterrent), I will acknowledge the possibility that a person innocent of capital murder could be executed. On the other hand, opponents of the death penalty (or those, such as yourself, who want certainty in its application) must acknowledge that without the death penalty, more innocent people will be murdered.
How does someone stashed in a prison cell for the rest of his life managed to murder any more innocent people than someone put to death by the apparatus of the State?