Right. So, revenge. An eye for an eye.
If we want to explore the moral aspects of this, how moral is it to imprison someone for the rest of their life?
What standard of care should they receive in prison? Is it moral to deny them things like Playstations, TVs, enjoyable meals, education, work, recreation, sex, yadda yadda. Is it moral to deny them any prospect of a normal life and to do the things people on the outside world can do. Is life in prison not also revenge, unless we provide them with sufficient stimulus to be "happy" or "fulfilled"?
If it is punishment and therefore happiness is not a factor, is this not revenge? When is punishment not revenge? Does punishment ever aid/enable rehabilitation?
If this is not punishment but just intended to achieve segregation away from polite society, then shouldn't you believe they have the right to happiness and fulfilment?
Where do you weave the moral thread in this complex web of criteria and objectives?
Personally, I don't think death is ultimately about revenge. In a world where guilt can ever be clearly established, it offers a solution that is quite ethical to all parties involved.
e: expanding on this a bit more, people have already said that punishment does not prevent crime as criminals do not consider the punishment when deciding to commit the crime.
Therefore punishment can only serve the purpose of revenge if you accept that argument.
Therefore if you take away any freedom or opportunity from "segregated" criminals (read incarcerated) then you are punishing them purely for revenge.
Can't argue it both ways.