Yes, as I wrote earlier it is theoretically possible.
That being said, the subjective here is subjective perception (what you see, hear, ...), not subjective evaluation of that perception. So IMO perceiving that someone shot someone else without seeing what preceded that absolutely does not give you the right to shoot immediately. Objectively evaluating that perception, it could be a murderer, or self defense, or an undercover cop. You do not have the justification to fire unless you see them threatening you, or someone who you actually perceived to not be a threat.
The way I see it, appearing threatening goes with carrying a gun. If you choose to carry, you need to be responsible for your appearance to the surrounding. As an example, aim a gun at a cop and it does not matter whether it is intentional, unintentional or even outside your control due to a medical condition. You will likely be turned into swiss cheese. It is your duty not to point your gun at people. The duty comes with the right to carry a gun. If you are unable to do so, maybe consider not carrying.
Also, I personally like how many European nations only allow concealed carry. This way, you don't create tense and possibly dangerous situations unnecessarily. You only reveal your weapon when you intend to use it.
Finally, what is the alternative to subjective perception? Oh, the terrorists gun was not loaded. You had no way to know but you go to jail, because objectively he was not a threat? That does not make sense.
Both subjective and objective evaluation of your subjective perception is the current requirement and IMO the reasonable one.
Of course, there are always details that could be improved.