Any LEO will tell you that the third best weapon is your voice.
First, I will assess the situation based on as much visual information as I am able to gather in a few seconds, and then make a rational (quasi-rational) decision.
Second, I will produce my weapon and adopt a strategic position and stance.
Third, I will yell "FREEZE!" at the top of my lungs, and then instruct the BGs---as loudly and aggressively as I can---to drop their weapon(s) and move away from the Deputy.
If you have ever watched the program "COPS" on T.V. (real footage, BTW) you'll know that two out of three times the BGs will vacillate for a moment, and then either attack YOU, or attempt to flee the scene. What you should do in one or the other case is obvious, and I will not go into that (assuming that you fully understand your rights and the laws of your country and state). What matters here is that you intervened and prevented the death of an officer of the law. You made a difference.
CMichael: your posts would have earned you multiple infractions at The Red Zone
http://www.recguns.com/TRZ.html
What, I ask myself, would you do if the Deputy had been your father/brother/son/friend? He most assuredly is
someone's. And what if the BGs come at
you with the Deputy's gun? Will you have time to peel out? to
run and hide? How will you take your chances, citizen? Tsk, tsk.