In some states out west, we have a legal situation called "Fighting Words." i.e., ac hateful verbal attack CAN be a defence for non-lethal use of force.
In this case, if the deceased had told the attacker to back off, and leave his GF alone, failure by the attacker to do so would most certainly provide a moral justification for a non-lethal physical response.
I would certainly shove, or punch out any jack-wagon that was verbally assaulting my wife if he failed to respond to my threat.
Much of our law is based on the premise of what would a reasonable, prudent man do?
I think any reasonable prudent man will come to the defence of his loved ones when they are being assaulted, even if only verbally.
You are distorting the caselaw here--the Supreme Court case involving "fighting words" was Chaplinsky v. NH where a person was charged with disorderly conduct BY THE POLICE for calling someone a racketeer and a g_ d_ fascist in public. There is not a doctrine that allows people to use physical force or threaten to do so against others for insults, even if those insults are directed at loved ones, and have oneself protected from assault and battery charges. Fighting words dealt with an exception to the 1st Amendment on disorderly conduct charges where a defendant's words were likely to cause an immediate breach of the peace.
Chaplinsky was charged with this and his defense was that the First Amendment protected his right from being arrested for mere words. The Supreme Court, during WWII, declined to overturn the conviction because it supported the police in that the particular words uttered could have spurred a fight. Had someone punched Chaplinsky in the nose for those words, that person would have been charged with assault--not given an affirmative self defense. Thus, fighting words, words intended to cause an immediate breach of the peace, are not protected from the police--that does not apply to individuals having the right to punch someone in the nose because their lady was insulted, called names, etc.
Self defense gets a bit sticky when you are coming to the aid of others and depending on just exactly what was said etc. and did the victim hear it, etc. In other words, you have to use the
same reasonable force calculation for the victim in beginning the physical fight that the convicted guy used to reply with lethal force. You got it right that someone could reply with words like back off, I gonna get security if you don't stop hassling my old lady. It would not be legal to say I am going to clean your clock. Likewise, itt is not legal to punch someone in the nose or shove them etc. in response to words unless the content and other circumstances of those words indicate that the other person is about to launch or did launch a
physical attack in response to words.
Quite a few people appear cavalier about punches, throwing someone to the ground, etc. but each of those could and have had lethal effect if the confrontation went badly. I can give you several cases where just that event has happened, particularly in bar fights. If the convicted guy fell and hit his head/died as a result of being shoved then the victim would have been in prison for manslaughter unless he could prove self defense to the satisfaction of the judge and jury. The motto sticks and stones may break my bones but words will never hurt me comes to mind.
Could the deceased have legally used force to repel an assault (not insults, not yelling, etc per se)? Possibly, but a reasonable objective belief that the person threatened to use force against another--loud words, leaning into the car, balling one's fist, etc. that would signal an imminent attack. One might or might not be considered reasonable by the jury.
A guy loudly threatening to call the cops on a girlfriend or even calling her names for parking in a handicapped parking place but otherwise not showing any signs of an imminent physical attack-not so much. You would be the aggressor in the second situation. From the brief survey of news coverage on the case, it appears mostly to concentrate on the video rather than the altercation that began the sequence of events and reading the previous posts have not brought out what was said, what was implied, etc.
Now, could a police officer have arrested the guy for inciting a breach of the peace--maybe. That would depend on a whole lot of factors primarily dealing with what was the content of the speech. It would not be considered disorderly for example to threaten to call the cops on the woman, even if the person was yelling and using a few cuss words. Chaplinsky has not been overturned but a later case, Brandenburg narrowed the range significantly for disorderly conduct charges to define unprotected speech as words that threaten AN IMMEDIATE breach of the peace which also applies to context in which the speech was delivered.
Moral of the story, do not plan on using physical force or even threatening such against those insulting the ones that you love unless you can prove to a jury's satisfaction that the person was committing an assault or in the process of doing so. You might be betting your life on it.