The majority of people who support capital punishment are of the right wing, and many of them of the religious right. Put yourself in the position of the person who is responsible for killing someone. Does it make a difference if they are innocent or not, if they are an enemy combatant, if they are the person who killed your child? It's potentially your karma on the line here.
You are either pro-life, which means you are against killing other people, or you are in arbitrary semantic land where words grant simple conveniences.