Kobayashi Maru...Time to break the rules.
Getting out maneuvered is one thing. Out maneuvering yourself and allowing an attacker behind you is another. In this case I see no option but to apply the final tap to the downed attacker since he may still pose a threat.
I'm not certain I'd be able to survive it in court, but at least I would be able to present an argument. There would most likely be a number of reasons for that final shot and if articulated well to the right jury, I might be able to explain why I did what I did so as to create reasonable doubt. I would have a 1:12 opportunity to convince them of the necessity of my actions. All I would need is one.
I'm not advocating summary executions or dramatic self sacrificing misplaced heroism. I'm saying that in this particular situation I would feel that neutralizing that threat could be necessary and justifiable to ensure my own safety and the safety of those innocent persons around me.