While I don't dispute that there are cut and dried cases where the death penalty could be applicable, there are still too many cases where the justice system is fallible. 99% positive, 'it was 'im what done it, guv.....' isn't good enough for me, not when there's that one per cent niggle.
Probably because we knew a few of Derek Bentley's relatives when we grew up. Also you need to look into the case of Timothy Evans who was also hanged in error, he was accused of two of the Rillington Place murders. Turned out to be John Christie.
However, that doesn't take away the fact that what these two have been found guilty of doing is utterly, utterly, reprehensible.
They should never be set free.