Going with the "BG has a knife" theory.....
First thing I'm gonna go is check to make sure I can take the shot, if necessary, and not hit an innocent bystander (depending on my distance from BG).
Question to self: Is he within 7 yards of myself or any other innocent person? If so, I don't have time to yell commands. I only have time to draw and make the shot.
If there's enough time/distance, the things I'd yell would be:
1) STOP! PUT DOWN THE WEAPON! DON'T MAKE ME SHOOT YOU! DO IT NOW!
2) SHOW ME YOUR HANDS! DO IT NOW!
3) LIE FACE DOWN, ON YOUR STOMACH, HANDS OUT TO THE SIDES AND PALMS FACING UPWARDS! DO IT NOW!
I'll continue to repeat command #1 until he complies. If he advances towards me or any other innocent person and breaks their 21' margin, I take the shot (if he's not already within 21 feet, to which the shot will already be made).
I will also be on the cell phone with 911 ASAP!
However, like other posters have stated, there's a lot of things to take into consideration. No scenario is "One-Size-Fits-All" and things can vary and change VERY QUICKLY! There's the matter of striking innocent bystanders, as well as what your local/state laws state about intervening in this scenario.
As far as the "Impersonation Of A Police Officer/LEO" thing goes, I'll give this advice: NEVER, EVER, AND I MEAN N-E-V-E-R, even utter the word "police" during the event. If you're looking for someone to summon police assistance for you, then yell "Someone call 911, NOW!" This should keep you cleared of ANYBODY (BG, victims, witnesses, etc.) assuming you as impersonating an LEO, because you never spoke the word "police".
Just my $.02 here, after reading all the replies thus far. I'm sure someone else here has a more intelligent, thought-out approach, which may reinforce, better suggest, or even negate my input. I welcome such replies, and hope to learn something on this "What If".
Always ready and willing to learn,
38SnubFan