In practice, that's not very helpful.
For instance, there's a suspicious guy lurking in your enclosed condo complex, which has seen its share of crime. You follow him, call 911 on him, and then you lose track of him. So, you get out of your truck and try to see where he went so you can update the responders when they get there (real soon now). Suddenly, the guy pops up out of the shadows and pins you to the ground and starts banging your head against the pavement. During the struggle, he realizes you have a gun. But you grab it before he can, point at his chest, and boom! He dead!
So. When did the "argument" start? When the "intruder" got pissed off that he was being profiled by da Man? Or when the intruder jumped the shooter, figuring he was going to mess up da Man for once?
Answering those questions is precisely what we have juries for.