Most people who commit crimes aren't caught. Around 38% of murders remain unsolved. Most of those who are caught aren't brought to trial. For obvious reasons,

Most people who commit crimes aren't caught. Around 38% of murders remain unsolved. Most of those who are caught aren't brought to trial. For obvious reasons, prosecutors strongly prefer cases supported by very strong evidence. The corollary is there are a lot of murderers walking around unremarked and free - either caught and released for lack of evidence, or not caught at all. So it's unlikely the death penalty has much a deterrent effect. There would be a much stronger deterrent effect if there was some magic way to increase the catch rate which didn't also intrude on privacy or civil liberties.