Comment Self defense (Score 1) 392
Possibly if they where really a life threat and you had a license or something.
Don't set a trap(Fails even in the USA), shoot them in the front, and have a 'legitimate' reason to express fear for your life. Them having weapons of their own(knives should work) would help.
If you DO end up shooting one in the back, being able to state that you were aiming for his buddy that hadn't turned yet might work. Or 'he turned as I pulled the trigger' while sobbing or something.
Remember, human reaction speed is something like 200ms. If you decide to pull the trigger a moment after he decides to turn, it's quite possible that you'll have pulled the trigger before you recognize that he's turning, too late to recall the 'pull trigger' impulses. And while guns fire fast, they still take time, giving him time to turn a little more.