This is the same argument as there may be valid reasons to curbing free speech. Once you start going down that route, there is no end. You say death penalty is

This is the same argument as there may be valid reasons to curbing free speech. Once you start going down that route, there is no end. You say death penalty is reserved for the worst of the worst and only if we are 100% sure they committed the crime, the next one in line would say it may be suitable for more and when we are 90% sure of the crime. How come it is different from Sharia law, which demands a hand for theft and eye for infidelity - some can say those are also justified. All punishment that's irreversible and violates one's body should be banned. On your second argument - if death penalty had the very least effect of turning people away from crime, USA would have much better crime stats than "rest of the western world". Most criminals operating at that level (murder, rape etc.) operate on the assumption that they will never be caught.