On top of this there is no point arguing whether it is a deterrent or not.The fact that there is a law against murder is enough to stop a law abiding citizen.
The argument against deterrence would therefor follow that since a person broke the law,the law wasn`t a deterrence so we must get rid of the law.
There is no way to prove a negative anyway.You would need people to admit that they would have killed someone if the death penalty was not in effect to know if it deterred them or not.Very unlikely to get an honest answer to that question.
The death penalty deters 100% of those who receive it from ever killing again.
That said, I would compromise on the death penalty, if the left would compromise on abortion.
In a way, all laws carrying punishment for wrongdoing (from death penalty for capital murder all the way down to fines for speeding) are intended as much to deter others as it is to punish the wrongdoer. Yet, in spite of the knowledge of the penalties, people still break laws at every level.
Does the fact that, in spite of harsh penalties for bank robbery, people still rob banks, mean that we should abolish prison time for bank robbery?
We can look at deterrence from different viewpoints. Obviously if no penalties, or very minimal fines, were imposed for drunk driving or speeding, how safe would the highways be? More safe, the same, much more dangerous? We know that driving would become far more dangerous and deadly if drunk and or speeding drivers were only cautioned or had nothing done to them. The death penalty is a deterrent.