Murder is the taking of another human being's life, usually premeditated. That usually gets a second- or first-degree murder in most legal systems.
Manslaughter is the taking of another human being's life, usually unintentionally or without control of your emotions. Examples: vehicular manslaughter or reckless homicide.
Killing someone who has a gun pointed at you or a fellow soldier in combat is obeying a lawful order. Shooting up Iraqi villages and executing civilians is a warcrime and every soldier in the US military has an obligation to disobey such an order and report it up their chain of command.
Killing in self-defense can either lessen a sentence or get an acquittal for the action. You as a human being have a right to your life, liberty, and property. This is the basis for the patents system, and why theft is just as illegal as murder. You have a right to protest legislation that aims to take your liberty, and you have the right to defend your life.
Honestly, when I see a religious "justification" for state-sponsored execution or any murder, I dismiss it as some of the most backwards barbarism I've ever heard. All Middle Eastern religions (hell, nearly all religions) teach three basic things:
1. Don't be a dickhead to other people.
2. Don't be a douchebag to other people.
3. Be nice to everyone.
The other 99% of stuff in the Bible, Torah, Hadith, Koran, etc. is all just bull**** added in by people later that starts wars and "justifies" racism, homophobia, and general douchebaggery. We really could be better off without it all.
Religion should be your own personal connection with a higher power. It should never be about telling other people how to live their lives.
Basically, all I see here is people arguing and *****ing over the bull**** parts of the Bible. And before you say it, this is not me having a pick and choose approach to Christianity. This is Christianity. Jesus died for my sins, and told me not to be a douchebag to others. I acknowledge and follow that.
I don't believe God told me to hate homosexuals or believe state-sponsored execution is right. Believe me, I have my non-religious reasons for supporting capital punishment.
Sentencing someone to death should never, ever, ever, ever have even the remote justification of "God said it was OK."