Moq is correct in noting that some of those sentenced to death are not guilty of the crimes for which they’ve been convicted. This happened so many times in Illinois (most notably to Cruz and Hernandez, who were convicted solely on the basis of a false confession and circumstantial evidence) that the governor declared a moratorium on the death penalty in that state.
One effective deterrent to crime might be a true sentence of life without parole, one that is not reduced due to prison overcrowding or good behavior.