Except that the guys in the scenario didn't use deadly force absent a threat. The very fact that there was an "exchange" of gunfire shows that.
Show me a case where two guys shot a running robber who threw down his gun and we'll talk. *Your* case has two citizens being fired on by a known felon. I'm doubting that even your prosecutors would have a problem with self-defense in that situation.
"To spit on your hands and lower the pike; to stand fast over the body of Leonidas the King; to be rear guard at Kunu-Ri; to stand and be still to the Birkenhead Drill; these are not rational acts. They are often merely necessary." Pournelle