Many years ago I was watching an episode of that TV news show, 60 Minutes. Ill never forget it. They were interviewing the reformist president of some African country. I forget which country.
Anyway, crime had declined dramatically since this man became president. The interviewer asked him why that was. I suppose the interviewer expected to hear something noble like We reformed our society, from top to bottom.
We killed all the bad people, the president actually replied.
Yet another real world proof that capital punishment IS a deterrent.