To me it's a simple moral question. It's never right to take another persons life under any circumstances other than self defence.
In any event, so far as the death penalty for murder is concerned there are enough cases before 1964 were people were hanged and subsequently shown to be innocent. In at least one case the victim, Derek Bentley, was actually cleared by the Court of Appeal.
There are any number of cases post 1964 which seemed open and shut. Stefan Kisko is one (standout) example but there are plenty of others.
Had the rope been available in the seventies there would be something approaching a dozen Irishmen and women hanged where (at its mildest) reasonable doubt was subsequently found to exist.