When can you legally use a gun against an unarmed person?

The notion that a defender may use a firearm in self-defense only if they themselves are faced with a firearm is entertainingly naive, but has no basis in actual law, nor in common sense.

In the eyes of the law a gun is not some magical talisman of power, it is merely one of perhaps an infinite number of means of exerting force. Legally speaking the law tends to divide force into two broad buckets: non-deadly force and deadly force. There is some stratification in the context of non-deadly force–a poke to the chest is not the same degree of non-deadly force as a punch to the face–but really none whatever in the context of deadly force. Deadly force is simply deadly force. For purposes of conciseness, I limit this discussion to cases in which deadly force is involved, as was the case in both Ferguson and Zimmerman.