When I was in high school...
...we had an assembly where a retired warden of a prison spoke to us about the history of capital punishment. He had presided over scores of executions. He was passionately against the practice, and had several personal horror-stories to tell. I was profoundly moved.
If I were to run into some data that did not sound like spin, and that showed that CP detered capital crime, then I would change my stand. I haven't run accross it yet.