There will be plenty of people who will try to equate "opposition to death penalty" with "bleeding heart liberal wants to set all child-molesters, rapists and murderers (proven or otherwise) free".
Uh...no. That's all sorts of logical fallacies right there, not to mention being wrong.
First, in my view, such convicts do need punishment. They are not good people, they don't deserve to live in society. But if they are dead then they cannot be punished any more. We can argue about what is cruel and unusual, but they need to be alive to be punished. Unless you are going to desecrate corpses which sounds fairly icky and antisocial, not to mention medieval. So execution is not punishment, it's a release from punishment.
Second, as has been mentioned many times, capital punishment is a one-way street that allows for no errors of judgement, no mistakes. Plenty of people have been convicted of horrendous crimes, including confessing to them, only to be fond not guilty many years later. Alas, some only long after they had been officially executed. Someone in jail can be set free and pardoned. Someone executed cannot.
Lastly, execution is not a deterrent. It really is not. The prevailing attitude of offenders is "may as well be hung for a sheep as a lamb". And then they go on to commit unspeakable crimes knowing they will never be punished for them really. It's the equivalent of the defiant "You'll never take me alive, coppers!" followed by the killing of hostages and bystanders while a blazing gunfight ensues. Did the death penalty have any effect on this sort of crime? It would seem it has never been a factor for consideration.