I will always oppose the death penalty as an institution.
It is not because it is probably more costly to sentence a person to death than it is to sentence them to life in prison without parole. I haven't been able to find a definitive answer, but most sources point to the fact that it is more expensive. But that's not why.
It is not that I find the death penalty barbaric. I do find it barbaric but I also understand that standards of justice differ between people as much as between cultures.
I oppose the death penalty because it is an optional sentence.
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As I said, the death penalty is optional. It may be on the table, but there are no laws requiring it in X circumstances. And any time that a consequence is optional rather than automatic, someone must make a judgment call as to which is more appropriate.
And when people have to make judgment calls, they will be influenced by their own lives, their understanding, and of course, their prejudices.
In the US criminal justice system, the person making that judgment call is a judge (how etymologically appropriate). Consider that in August 2009, 70% of federal judges were white men, 15% were white women, 10% were men of color and 3% were women of color.
Obviously in a situation with one racial-gender group so outweighing all others, system-wide prejudices will become evident. I doubt I need to go into that.
But even if the judges accurately represented America (that would be about 32% white men, 33% white women, 17% men of color, 18% women of color), even in a perfectly representative environment, you will still have individual judges with prejudices. These prejudices may affect their decisions, and thereby affect the defendant's lives. Racial bias is my biggest concern, but it could even be as non-historically rooted as a judge unconsciously disliking a defendant because of a passing resemblance to an ex-spouse (I'm not a psychologist, but you know what I mean. It could be anything.) So having a more diverse set of judges will NOT eliminate the problem of unfairly applied death sentences.
Of course, it is tricky to eliminate those prejudices' influence on sentencing, e.g. how can we find out whether this crime is really worth 5 years or 10 years given all the details of the case? That might be nearly impossible.
But to kill an convicted person is such a cut-and-dried case; there is no relativity to it, since you can't be relatively more dead than the next guy who committed the same crime. To kill a person is all or nothing.
In that situation, a single judge can create an 100% different, unambiguously unequal consequences for defendants with similar crimes.
Compare standing on two feet and shifting your weight from foot to foot, to sitting in a chair. When you are standing, the weight per foot might be shifted around and unequal, but it is categorically different from sitting in a chair. So it is with a death sentence.
It is one thing to say that it is right and moral to execute a cold-blooded murderer. But it is statistically impossible to guarantee that this type of justice would be carried out uniformly.
When death is on the table, the stakes are too high for even the smallest deviation from perfect and uniform justice.