My attitude is when I am carrying a handgun, every confrontation by definition is an armed confrontation by virtue of the fact that I am armed. We can never lose sight of the fact that being armed changes everything.
Someone points a taser at me, I am shooting to stop the threat.
As srothstein points out, the justification is clear for a police officer. When disabled, they can get shot with their own handgun. Unfortunately, cops getting shot with their own handgun happens far too often (one time is too many).
Whether my handgun is visible or not makes no difference. I've seen too many CCTV videos where the perp cold cocks someone. In the act of looking for money or wallet, they can easily find the handgun. Or, simply the act of falling over could disclose the handgun in some way (even if only printing). Either way, if I am out, I am vulnerable to getting shot with my own handgun.
There are the scenarios where force is presumed to be reasonable, such as "aggravated kidnapping, murder, sexual assault, aggravated sexual assault, robbery, or aggravated robbery".
However, there is also the generalized justification defense under PC §9.31, specifically ""a person is justified in using force against another when and to the
degree the actor reasonably believes the force is immediately necessary to protect the actor against the other's use or attempted use of unlawful force." Looking to PC §9.32, then I will argue that deadly force is immediately necessary since the possibility of having my own handgun used against me makes the Taser potentially deadly force. Remember, the standard uses the phrase "the actor reasonably believes", and I think this line of thought is reasonable.