BT: Really! Did you read the Teshuva inside?
He says it would only be for murderers who kill to disrupt the fabric of society AKA someone like Son of Sam.
As you quoted: then a different system of law was utilized that was concerned with the pragmatic question of stopping killing and the goal became saving the society.
R’ Moshe says (in the part you left off) that otherwise Capital Punishment should not be implemented! Due to lack of a Sanhedrin (as I said originally).
Please quote entire teshuvos, not just those parts that you believe help your case.
Besides, how are you getting the english translations so quickly? If you are copying, please source (besides, it would help us who have to read it in the original hebrew!!!)