Deterrent, revenge, justice, rehabilitation are all words that people can twist to suit their support or disagreement with the death penalty.
I have a strange view of the death penalty, im not against it for any moral reason, there are people who commit crimes so heinous that its possibly a fitting punishment. However humans are prone to making mistakes, so eventually some poor guy will be executed, who would been innocent, so I dont think its right to be used.
Prison isnt a deterrent for plenty of criminals, they still commit crimes. 99% of people wont think about committing a crime that could result in a death penalty, somehow I dont think the risk of lethal injection, prison etc played heavily on the minds of Tsarnaev, or the previously mentioned McVeigh etc
Tsarnaev probably deserves to die, Im glad its not my call though, I doubt Id be comfortable making that decision.
As Mindimoo also mentions above the glorification of takings someones life is incredibly weird to me, the scenes when Bin Laden was executed, Hussein was hung etc, these just seem incredibly wrong to me. But I say that as someone unaffected by their actions.