There are very few cases which are beyond reasonable doubt and how can you decide which are and aren't? As I've said before innocent men have spent 20+ years in prison for crimes they didn't commit, innocent men have been put to death in the uk and pardoned 50 years later.
Think how stupid the average person is, then think on a jury there is a good chance of third of them are even stupider, would you be happy with these people deciding on your fate?
There are innocent men and women in prison as we speak that people think did the crime beyond reasonable doubt. There are 4% of death row on average who are innocent who have been convicted beyond reasonable doubt.
Saying we'll only kill people who are guilty beyond reasonable doubt isn't a very good argument because beyond reasonable doubt can be wrong. Ched Evans spent a few years in prison because he was guilt beyond reasonable doubt, how did that end up?