gavra: Rav Moshe wrote this for the Governor of New York to encourage the implementation of the then proposed death penalty for first degree murder. It was applicable to more than Son of Sam; including non-serial murderers.
When there is 15,000 – 20,000 murders PER YEAR (compared to a Jewish Beis Din having ONE death case per SEVENTY [or seven] YEARS) in the United States, you definitely are in an emergency situation that requires saving society, is what Rav Moshe is saying. That’s why he wrote this.