No offense, but I don't need the Bible to tell me why a person who has been executed is not able to commit future acts of violence.
Seems like an obviousness that is pretty glaring.
The death penalty probably is a weak deterrent to others committing murder but prevents murders that would be perpetrated by some of the executed killers were they not executed, in prison and out when they is pardoned or given parole or such. To deter other killers something a bit more excruciating is required, crucifixion or Hanging, Drawing, and Quartering, perhaps. Even that would likely provide only marginal deterrence