For a relatively brief period of time, about twenty years ago, I was opposed to the death penalty on that basis. One night, then living in Connecticut, I was watching a New York City news telecast that covered the mass murder of something in excess of ten (14, I believe) women, children and infants in Brooklyn or the Bronx in a storefront converted to a residence. The victims were the family members of some Dominican Republic native narcotics dealers. The perps were rival narcotics dealers. The youngest victim was a six-month old infant machine-gunned while crawling through broken glass, debris and his/her mother's blood. I pondered the news story for about ten seconds and realized that a society that refuses to impose the death penalty on the perps of such crimes (NY had no death penalty at the time) has little business existing.
The infant could not have even been a witness against the perps. The infant's murder (and probably the rest of the murders) was pure, shocking, gratuitous, unadulterated evil of a sort that requires death as a penalty. I have not wavered on the death penalty since. I do think that the perps of such murders would be quite well advised to seek the forgiveness of their Heavenly Father before execution. They have plenty of time to repent.
That's a vivid story! It is ironic that murderers sentenced to death have time to repent but very often their victims do not. The law simply must be designed to deter as many murders as possible, and that requires capital punishment.
When the law elects to allow a murderer to live, or a jury votes to let one live, the law has simply elected someone else to die in the murderer's place.
I'm not sure that is a just outcome.