He first shot the unarmed assailant, letting the armed one get away, then proceeded to stroll around the shop (thus making clear he himself didn't consider the downed assailant to be dangerous) before executing him.
I'm willing to bet he didn't choose to ignore the guy with gun and shoot the one without one. The first part of the shooting was completely legal and appropriate.
The second part was wrong and illegal. But, no, it was not 1st degree murder.