You can only shoot someone in self defense if they're threatening to shoot you with a gun or they're trying to stab you or use some other weapon as deadly force.
You can't just shoot a person in self defense if they're just trying to fight you and you have to be sure that you really have to shoot the person with the gun because you'll have to prove that in court and have a reasonable reason that caused you to use deadly force as in shooting the person.
Only shoot someone if absolutely necessary and if any other options won't save you from the person that is trying to attack you or trying to harm you in anyway.