A death penalty that is never carried out is the same as no death penalty. I can't even call that a nice try.

The Biblical commandment you reference is more correctly translated "You shall not murder." The Bible recognizes the state's authority to "wield the sword" or carry out punishment and in fact prescribes death as the punishment for murderers. So neither your premise nor your argument hold.

The death penalty is a valid deterent and punishment for murder. Whether it should be applied in Roof's case I will trust to the jury, who have heard all the evidence presented. Anyone else is an armchair quarterback with someone else's life on the line.