nanaej
Ann, I would also like to add the names of Timothy Evans and Derek Bentley to the list of hanged men who subsequently were shown not to be the murderer.
As a preventative measure the death penalty does not have much impact. My father, a defence barrister, said that many clients and cases he knew did not consider the potential penalty when committing murder. Numbers on death rows in US would also support this.
Your statement that the death penalty does not prevent murder is flatly contradicted by the statistics I quoted. The US system is different, where people can spend a decade or more on death row and so the link between crime and penalty is weakened.
Timothy Evans would not have been convicted today because of DNA. But you are wrong to say that Bentley was 'shown not to be the murderer'. He was hanged under the rule then prevailing that if a gun was used fatally in the course of a crime, all those involved in that crime were deemed to be guilty, not just the man who fired the gun. It was known from the outset that Craig fired the gun, not Bentley. There is no need to reintroduce the same rule with the introduction of the death penalty.
Obviously, nobody claims that capital punishment eliminates all murder, and so those who murder despite its presence will always have failed to take it into account. So your father's experience is entirely to be expected.