“Reb Moshe zt’l said that we shouldn’t support capital punishment because this is going to cause killing unzerer.”
There is a tshuva in Igros Moshe to the “governor” (dosent say wich governor or what state) in response to that governors inquiery to the Torahs point of view in regard to capital punishment. His answer is that the Torah has very strict requierments and was carried out very rarely.