This is what Rav Moshe wrote to the Governor of New York to encourage the implementation of the then proposed death penalty for first degree murder –
As a consequence of these two factors there were almost no Jewish murderers because of the awareness of the severity of the prohibition of murder and because they were educated by means of the Torah and the punishments of the Torah to understand the seriousness of the crime. They were not simply afraid of punishment in the sense of getting caught but were afraid of the crime itself.