The death penalty, like any other form of punishment, was never meant to be a deterrent; it was meant to be justice.
What does "justice" mean in this sentence? Two wrongs make a right?
I see three possible reasons for punishment.
A) Vengeance.
B) Rehabilitation.
C) Protection of the public. Both deterrence and isolation.
Aside from Vengeance, I see little use for capital punishment. There are some people who could continue causing death and mayhem, even incarcerated. El Chapo comes to mind.
But mostly, locking people up is cheaper than providing adequate protection from wrongful execution. Execution isn't much of a deterrent, because most criminals don't think they'll get caught. Spending the resources on more effective policing will probably result in fewer innocent people dying than spending it on lawyers.
So I oppose capital punishment because I think it's almost entirely about vengeance, which I consider an ugly emotion.
Tom