The other day, I mentioned parenthetically that I would get to me reasons for opposing capital punishment in another post. Ere I forget, I'm writing that another post now. I have two main reasons for my position. One is moral/philosophical, and I might get to that in another post. The other is legal, and that's what I'm going to talk about in this article.
There are two kinds of rights we have in our legal system, substantive and procedural. Substantive rights include what we usually think of as basic human rights (rights not to be tortured, to worship or believe what one will, to vote in elections, and so forth) but also can include special rights such as those gained under a contract or by statute; under a contract, if you do your part, you have a right to what the other party promised. Substantive rights can be disputed at times, and won or lost under various circumstances.
This is where procedural rights come into play. Procedural rights are rights to a fair process (hence the name) by which substantive rights can be determined. So, for example, you have a procedural right to a fair trial to decide whether or not you lose your substantive right not to be imprisoned.
Procedural rights are interesting because although we usually speak of them as belonging to the accused (in criminal matters) or to the litigants themselves in civil matters, they are actually more diffusely owned by society as a whole. That is, we all have an interest in procedural fairness, and we all lose if our procedures become unfair (even if some of us happen to benefit from the unfairness). It's just that the parties to the dispute are the ones most immediately concerned with the process, and thus most likely to zealously assert those procedural rights. Besides, if one party should choose to waive those rights in a given case, no one else is in a position to complain about unfairness.
Now, while substantive rights can be won or lost, procedural rights ought to be absolute, in part because they don't just belong to the accused. There is no crime so heinous that we would want to say you lose your right to a fair trial; no matter what you're accused of, you have a right to insist that the state prove its case against you before it can mete out punishment. And society also has a right to insist this as well, because (1) none of the rest of us want the state to be able to punish us without proof, and (2) society has an interest in making sure that we've got the right guy; it wouldn't do to convict you while the real culprit goes free.
My legal problem with capital punishment, then, is not that it takes away substantive rights, but that it extinguishes all rights, substantive and procedural, absolutely and irrevocably. Once someone is executed, there is no possibility that new evidence can exonerate them, or that the law on sentencing could be changed, or that the Crown might grant a pardon. While I have considerable trust in the legal system in principle, part of that trust derives from the fact that it includes so many error-correction mechanisms, such as appeals, Acts of Parliament, and Crown prerogatives. Capital punishment deprives society of the opportunity to correct its errors.