To What Extent is Revenge Justified?

In Poe's short story The Cask of Amontillado,  the narrator Montresor murders his "friend" Fortunato. Throughout the story, Montresor never gives any concrete evidence that supports the horrendous act that was committed.  The only justification provided is "The thousand injuries" that Fortunato has given to Montresor. Even this does not provide enough evidence to justify Fortunato's death.  Even if the injuries were terrible, Montresor lived on after these injuries, where Fortunato perished.  I personally feel that someone who kills deserves to be killed, but since Fortunato never killed (as far as we know), he did not deserve what became of him.  This broached the hot topic of death sentences in my mind.  In 16 of the 51(including D.C.) states, the death penalty is not an option (to see which states, go to http://www.deathpenaltyinfo.org/states-and-without-death-penalty).  While some view the death penalty as barbaric, I view it as a viable choice.  In the case of homicide, I think the death penalty should be used in most circumstances.  I also think that the death penalty may scare off possible murderers.  Is the death penalty something that is useful, or something that has a negative effect on our country?