Too many people think a horn is for expressing your displeasure, when in fact it was only put there to warn other drivers of your presence when needed. After a close call of some sort is over, any horn use is wrong.
(No offense to OP)
Just like the old fable/ parable/ saying/ whatever, about the frog sitting in a pot of water on a stove, slowly realizing that it's getting warm, then by the time it realizes it's too hot, it's too late... Now compress the time line and apply that concept of an rapidly escalating threat and the consequences of not realizing/reacting either fast enough or appropriately.
Guys- re read the OP's post. Using a horn to warn an oncoming vehicle that's failing to yield and turning into your path is the reason we have horns.
Now, consider the rapid sequence of events- Per the OP's description, I'm guessing it probably took 2-3 (?) sec for the vehicle to come at him, panic stop then other driver get out of his car in a threatening manner, then another occupant does the same. Going from relaxed/ distracted driving to "Oh **** we're gonna hit" then trying to decide if you are facing a lethal threat, now two guys/ two decisions, look for weapons on or in someone's hands at night while looking for a clear path out is tough for anyone.
Ultimately, he broke away from the situation and no one was hurt, but when we read of shootings in the news, situations such as this are sometimes the back story to the actual shooting, be it "Armed thugs shoot man and women at intersection" or "Man shoots two armed/ unarmed men that descended upon his vehicle after near collision in an intersection". Could go either way.
Gas and a clear path of escape is ideal, but what if traffic or obstacles eliminate that option. What happens if the guy approaches your window? Do you wait for him to pound on it/ break it and attack you? Wait for him to pull a knife or pistol out? At what point does a rapid escalation of threat (though still no visible weapon) warrant pulling/ aiming and warning off or possibly shooting a person? Would it have been different if two "Thugs" descended upon a little old lady for her shopping cart blocking their exit from a grocery store? IIRC, in one of my past NRA mags, this happened and fearing for her life, she shot one of the two despite not seeing a firearm and was cleared by the DA and lauded for defending herself against the "attack" (Even though neither attacker actually made physical contact with her).
Easy to do the Monday morning QB thing here and calmly state what we "would" have done, but throw yourself in his seat for 10-20 sec and try to keep up with the pace of escalation, let alone think/ react ahead of it.
Come on froggie, no time to think, only time to do. Already too hot.
Except that's not what he did. "I lean on the horn to let individual know that was too close."
He "leaned" on his horn to voice displeasure with the other driver's actions...not warn him of an impending collision.
At no time was he presented with deadly force, or even the threat thereof. Reaching for a gun, instead of focusing on operating your 2,000lb weapon is foolish and gun-happy. He stated his firearm was already in an accessable location, so there was nothing else to do with it at that point...it was there if he was truly boxed in and one of the other vehicle's occupants attempted to enter his vehicle.
Luckily he did focus on operating his vehicle and drove away. It's not Monday morning QB-ing when the OP asked for input. He asked "Shoot or don't Shoot" and got an answer.
Driving your car into some one else's car is deadly force. Doing it accidently is an accident with no intent. Hitting some one's car with no regard for safety is an attack. Running a red light is life threatening. Does any of these factor into using your gun? Not after the danger has passed and their car is stopped. You still won't know what their intent was. Driving away was the only good choice.
Ultimately the OP made the right decision. That's what matters. I don't think their was any reason to even consider a gun; he did, but made the right choice.
In my limited driving experience, "horn leaning" by driver A is usually followed by "shooting the bird" by driver B... which is followed by.... well you get the picture.