Since many murders as done by criminal to criminal (gangs) or between people who know each other (this comes a recent NY city report) deterrence is hard to measure. Also coming into this is the common decades long time between conviction and punishment in these cases.

One thing I know is that the recidivism rate for those executed is zero, we can not say the same for those imprisoned and then released.

I am addicted to spell checkers and apologize for any errors