1. Murderers, by definition, disregard the rules of society. Executing a murder prevents (or "deters", if you prefer) their future crimes. Note that crimes in prison are crimes.
2. 4 rounds of .308 and a blank are cheaper than even a single day of prison.
3. No justice system is perfect, but I agree that it could be improved. Make prosecutorial misconduct a serious matter, with punishments similar to the punishment under consideration. We aren't exactly swimming in false convictions, but there are still way too many. With real consequences (prison, death, etc) for prosecutors (and maybe extend to official witnesses, aka police) sketchy convictions would diminish.
4. Most of us consider the time, place and method of death to be important. If we didn't, why would we consider murder to be a crime?
5 and 6. You must be the hippest atheist in the coffee shop.
7. Begging the question.
9. Murder is not a synonym for killing. Please consult a legal dictionary, or a 10 year old child, if you need the difference explained.
10. Nice of you to expand on #4. Still rubbish, but overly wordy this time.
11. Gun owners tend to be responsible and have a keen understanding of the mechanics of civilization. We'd be happy to be judges, if you'd really like to exclude the gunless from consideration for judging capital crimes.
12. Restitution is rarely possible even under the best of circumstances. For capital crimes, it isn't possible even in theory.