Thou shalt not kill is a mistranslation. It's more properly rendered "Thou shalt not murder."
Trying to find a good definition of murder that doesn't circle back to the law definition, but with limited success.
Murder is the unlawful killing, with malice aforethought, of another human, and generally this state of mind distinguishes murder from other forms of unlawful homicide (such as manslaughter).
Malice aforethought is the "premeditation" or "predetermination" that was required as an element of some crimes in some jurisdictions,
If someone's trying to kill you, it's not murder to defend yourself, even if you kill them.
Now, loopholes to this do pop up. In the Old West, for example, the shootout duels at high noon took advantage of that if someone drew on you first, you could retaliate and kill them.
From what I've heard about many of the sheriffs of the day, though, they weren't interested in getting involved when both people willingly went out into the street. If the guy who actually drew first managed to shoot and kill the other, then technically the sheriff would have to step in, but was it really as bad as when a guy just walks up and shoots someone else without warning?It reminds me of a story I heard about recently where a man found his 4 year old daughter being molested and beat the molester to death.
This wouldn't be murder, because you don't premeditate to find someone in this kind of horrible situation. However, this is definitely a case of exceptions where the laws need to be clearly written.
According to the story, no arrests have been made so far, and it's going to take a grand jury to decide whether they're going to prosecute him at all. A grand jury seems arbitrary, but how else would you even decide this kind of thing? Do we really want to have every law written out to the point where these extreme circumstances are explained in ink?
I'm not sure I'd want it spelled out when you're allowed to kill someone, other than to save your own life. I see a clear argument for both letting this man go and for punishing him, and wouldn't want the task of judging him.