The commission of the assault will be legally justified if our hypothetical actor reasonably believes pointing the gun to be necessary to avoid an evil which is worse than the evil sought to be prevented by the law which makes the assault illegal. In your hypothetical, the evil being prevented is an escalation of the event to the point where someone is going to actually use deadly force, and the evil which the assault statute is designed to prevent is the putting another in fear of imminent serious bodily injury by physical menace. Which is worse?
It would be hard to imagine a court finding that making someone afraid was not an evil worse than either the aggressor or the defender actually applying deadly force. Of course, if the Government can prove display of the weapon was not in fact “reasonably necessary,” the defense will fail. There are other conditions as well.
If our actor was either reckless or negligent in bringing about the situation under which the choice of evils was necessary, the justification defense will fail. There are lots of ways one might negligently or recklessly cause or provoke a situation, too many to try to imagine. But one warrants special notice.
Bear in mind that pointing a gun might actually precipitate an attack, perhaps from a third party our actor does not even know is involved. This might be the proverbial girlfriend with the .25 auto in her purse, who believes her loved one is about to be killed by our own hypothetical actor and she opens fire. It might be some well-intended but not very prudent CCW holder who happens by. If our actor precipitates an attack by a third party, and is then compelled to use deadly force against that third party, the justification defense may well fail. There may also be a civil suit of significant merit.
In addition, if there are any statutory exceptions to a justification defense dealing with your specific situation, the defense will fail. And if there is any “legislative purpose to exclude the justification claimed” the defense will fail.
Got all that?
Applying these rules to our hypothetical actor “displaying a weapon to stop an attacker,” the attack must represent an immediate threat of serious bodily injury or death. Otherwise, the “evil to be avoided” is less serious, and may not justify the commission of an assault. Is the attacker an agitated paramour trying to slap our actor’s face, or is there a straight-razor involved? Someplace in between?
Introducing a firearm into any situation will, in Pennsylvania, substantially increase one’s legal exposure in too many ways to list here. (One example is the application of our five-year minimum sentence requirement for specified crimes committed with firearms, including aggravated assault.) Both for this reason and due to fact the evil one seeks to prevent must be very serious to justify an assault, I am confident in advising that pointing or displaying a firearm is a defensive tactic that should be reserved for situations where one has good grounds to believe serious bodily injury or death will occur if they do not succeed in dissuading the attacker.
You have postulated our actor is pointing a gun “during self defense.” Let’s assume, then, the actor does hold an objectively reasonable belief that he or she is in immediate danger of suffering serious bodily injury or death, of being kidnapped or of being subjected to involuntary sexual intercourse by force or threat, to himself or to a third party who is or would be reasonable in holding a like belief, but the actual use of deadly force is not immediately necessary to avoid such injury or death.
Got all that, too?