What's the difference? Dead or locked in a horrible box for life? We euthanize sick cats; we don't make them suffer needlessly. I would argue life imprisonment is a punishment more severe than death.
Just by way of disclosure, I have witnessed two executions in Texas back when I worked for Reuters. I spent a lot of time on death row, interviewing prisoners as well as families, so I don't have a positive feeling about the death penalty... however I contend that life inprisonment is even worse.