Im with Bhallg2k, I had to write an essay on capital punishment a few years ago. At first I thought the same, murderers are useless and only consume space and waste resources.
Well then I found out that it isn't as if those sentenced to death are shortly thereafter executed, not at all. Such convictions go through numerous appeals that cost the state money and back up the judicial system. After at least a few years of that the convict sits on death row for another decade or longer before they are finally executed.