The problem is a more basic one. The armed citizen can use lethal force against an immediate, dire threat to his life or health or those of other innocent persons.
But short of lethal force, the average person has few options. Police have an escalating series of weapons, with lethal force as the most effective, but last choice. A police officer carries Mace, a baton/blackjack, a Taser, and a pistol, as well as the means of restricting a suspect's movements after arrest (handcuffs). He/she is generally allowed to use deadly force to prevent the escape of a person known or reasonably presumed to be a felon.
But a citizen who draws a gun, then tries to hold someone at gun point, has few options. If the "suspect" simply flips the citizen the bird and walks away, there is no justification for opening fire; the threat, if it existed, is gone. Only if the "suspect" has committed a felony in the presence of the citizen might lethal force be justifiable, and that is arguable. ("Citizens arrest" is a non-starter; few citizens know how to make a lawful arrest and an attempt to do so could result in a felon going free.)
The armed citizen can defend himself (whether carrying the gun is legal or not is another issue and irrelevant to the question of self defense) but has no powers of arrest, and no way of holding anyone short of shooting him. That is the basic problem, not where the trigger finger should be or not be.
Jim