I think it is unfair to expect me just to say that there is such a thing as universally objective morality as a starting point.
It's totally fair, given what we are talking about, which is the criminal justice system. Notice the "justice" part-- there is no justice without objective ethics.
This is my position-- if there are crimes that may justly cause the criminal to surrender his rights once convicted of the crime, then it is unreasonable to arbitrarily say that the right to liberty is one of them, but the right to life isn't (i.e. it's okay for the state, or anyone else, to deprive someone of the natural right of liberty for X crime by imprisoning them, but it's not okay for the state, or anyone else, to deprive someone of the natural right of life for ANY crime by imposing a death sentence on them).
In practice I do oppose the death penalty for a number of reasons, but the idea that ever taking someone's life in retribution for certain crimes is always unjust is not among them.
So if you think all morality is relative, there are no natural, universal rights, no universal concept of justice-- then why would I waste my time arguing with you about it, because you can always fall back on that? You might as well be taking the position the state has no right to punish anyone for anything, morality being relative and all that.
If you don't accept those presuppositions, fine, but there's no point in talking about it otherwise. It would be like debating the merits of Old Earth vs. Young Earth Creationism with an atheist evolutionist. Our starting points are so diametrically opposed that there is no basis for conversation/debate on the topic.