There are a lot of things to consider with the given scenario "can I shoot an unarmed person if they want to fight me?" You have to decide what you want to do and be prepared for the consequences plain and simple.
If you think doing 3 to 5 years hard time in a state penitentiary for manslaughter is no problem, then by all means shoot that unarmed guy who picked a fight with you because he feels you slighted him somehow.
State prison is full of people who made the wrong decision in when and under what circumstances to use their firearm. They aren't bad people, they just made a poor choice when they thought that using their gun would be on solid ground.
You can sit on the witness stand crying all day long saying how that scary looking guy who turned out was only two years younger than you and considered to be about average size and shape could have hit you in the temple and killed you... may have been able to strike you in the trachea and crush your larynx and all sorts of hypothetical outcomes until you are blue in the face. At the end of the day... the law and the courts "generally" frown on those who pull a gun and shoot an unarmed person who was foolish enough/drunk enough to pick a fight with someone they didn't know was armed.
Now, with that said, are there circumstances where shooting an unarmed person is excusable? Absolutely, but they are relatively rare and few in the numbers of situations. Your hypothetical explanations of "well he could have hit me here and that may have done this to me" is not going to be a satisfactory answer to a grand jury or a jury of your peers.
Getting a broken nose and some bruised ribs or a black eye or maybe a tooth knocked out is "generally" not considered to be a life threatening injury or even a serious crippling injury no matter how much you say that hypothetically you could have gotten a detached retina from the black eye, or you could have choked to death if you swallowed that broken tooth.
These are the facts... You have a gun! The other guy doesn't! You know you have a "stacked deck"... the other guy doesn't know you have a gun until you pull it out and shoot him with it! He believed he was fighting another unarmed guy!
Bullies, drunk or otherwise who like to fight people, don't pick fights with people they knew beforehand to have a gun! Therefore the prosecutor in the criminal case or the attorneys for his estate in the wrongful death case will argue the "unarmed victim" you killed may have been drunk, and may have been a bully... but he thought he was fighting a man "on common ground" and it did not warrant being shot down in a fixed and unfair fight! (May not be 1st degree murder, but probably good enough to send you away for a few years for involuntary manslaughter!)
The courts will hold you to a higher standard of care to avoid the situation, walk away and maybe even going to extrordanary means in not bringing your gun out even if the guy manages to hit you a few times.
Again, are there circumstances where shooting an unarmed person justifiable? YES! But you are on very shakey ground. If you are going to claim there was a "disparity of force" you should have some very compelling reasons that are clear and obvious to the court or grand jury. (Hypothetical explainations are really not going to be acceptable.) Disparity of Force is not just a loose term to throw out in court. You must be able to clearly show the factors that pertain to that disparity in order to convince a jury to excuse you for taking the life of an unarmed person. There has to be some accountability for taking someones life.
It is for that reason I strongly recommend people really understand the laws, how the court system works and know specific details of how lethal force is used and justified.
Massad Ayoob is internationally recognized as a preeminent authority of the legal of use deadly force in lethal situations for both law enforcement officers and civilians. To date, every single judge or attorney who has taken his 40 hour LFI-I Course will emphatically admit, they learned more about the judicious use of lethal force in his LFI course than in any law school in the country. And for good reason. The man knows his subject matter.
His school covers in depth what no other shooting school out there covers regarding the subject. I would highly recommend attending at least his 40 hour LFI-I Course. I think the course is $800 and is offered all over the country several times a year for those who can not travel to his home base in New Hampshire.
If you can not afford his course for a while or want a taste of what he has to offer, I highly recommend, and actually believe it is a "must have" item is to get his Judicious Use of Deadly Force video available in VHS and DVD for $34.95. It is over two hours of lecture straight out of his 40 hr. LFI-I course and it's value is truly worth your life! If you like what you see in that video I would also recommend Physio-Psychological Aspects of Violent Encounters which is another two hours straight out of the LFI-I course.
After that, I think most people will know exactly where they stand legally in most every situation and in every state of the union and most western countries of the world.
To visit his website and check out his courses and videos go here:
To purchase Judicious Use of Deadly Force and Physio-Psychological Aspects of Violent Encounters directly as well as many other books and videos from him and other authors, go here:
Everyone has their opinions and my post is only my opionion but I feel you will be more than happy with any of my recommended purchases from Ayoob.