The death penalty makes no sense.

[Amarok]

Time in prison doesn't necessarily stop their murderous ways

But it can 

 guards, other prisoners, and people outside of prison could be at risk if they still have sufficient authority to direct their murder or escape from prison.

Prison escapes are rare 

 Additionally, after they serve their time, some murder again;

A possibility they may murder again does not justify murdering them .And some don't so why should they die ?

 Mr. Moore won't be murdering anybody anytime soon which makes sense.

And he has no chance to be redeemed either and victims remain dead .So nothing of any merit was accomplished .

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Addendum:

Actions have consequences.  And in most likelyhood you will already know the consequences of those actions before you carry them out.

So, by going out and performing those actions you are accepting thise consequences.

consequences don't equal death or torture .