Vengeance is not justice.
I cannot see how it is civilised to shoot, electrocute, hang, poison, lethally inject or whatever "humane" way you put down people who 12 people have decided is not fit to live. Ok 14 if you include the prosecutor and the judge.
Surely it is more civilised to have a justice system where the perpetrator of this crime is gaoled for a determinate length of time, and hopefully re-educated. I nthe case of the abhorence that you mentioned Brian then life should mean life, also in the case you mentioned, (Willow is right they do happen here) once the animal has done one crime like that and knows they will be executed, where is the deterrent to prevent more happening? If you are going to die anyway why not kill a few more and make your mark as the new Ted Bundy. Capital punishment doesn't work as a deterrent it is only used as a tool of revenge.
Also as you raised the point of capital punishment as a deterrent could you also answer why if that is the case countries with the death penalty have more murders than those without?
NationMaster - Statistics > Murders (per capita) by country
Oh and just an aside my 13 year old daughter was raped 2 years ago and yes I wanted vengeance, after a while it dulls down and knowing the ***t is suffering in prison is enough for us, and I am pleased to say her as well.