The deterrent effect has a small impact on murder rates, but a substantial savings in innocent lives, as reviewed below.
The US has averaged around 33 executions per year since 1973, which equals a deterrent savings of innocents lives of from 33 to 924 per year.
1) a) "Death Penalty, Deterrence & Murder Rates: Let's be clear"
Top 25 Most Dangerous Neighborhoods in America, NeighborhoodScout
99.7% of murderers tells us "Give me life, not execution"