It certainly means you can call the cops and keep an eye on someone, and if they assault you and continue to threaten your life/health, you can shoot them.
The problem here is there were no witnesses to the beginning of the physical battle.
Or I guess a person that isn't all honest and squeeky clean could just pull the trigger, and then tell a little white lie about the "assault", or "physical battle."
SYG is a mess to me, but that's just my opinion. YMMV.