"Are you testifying that you executed my client without giving him the chance to surrender?"
Why would you be an idiot and be on the stand to testify? You don't have to testify and so that question never has to be posed to you in court. There is no reason to worry about that question being posed to you in court unless you want to testify.
Even so, your lawyer would then object to the line of questioning and the question would not be allowing in that form.
You are more than welcome to try a McKown.
I know of no statutes that require you to give a person trying to commit murder or mass murder an opportunity to surrender before you defend yourself or the lives of others. Maybe you have a law where you are that says you must give notice of offer of surrenderance before defending your life, but I have never heard of such a law.
Say you did give the shooter a chance to surrender (verbally) before you shot and killed him. Did you verify that he did in fact hear your offer to surrender and was capable of comprehending the rammifications of the offer and was in a sound mind so as to make a rational decision before you shot him? If the shooter was not an English speaker, did you make the offer to him in his native tongue?
Hell, the guy has been shooting people left and right. His ears are probably ringing. He probably will experience considerable auditory exclusion. He may or may not hear you. Even if he can hear you, he may not understand you. So even if you pose the offer of surrendering, how long are you going to way for his verbal or physical response before you decide to act and defend your own life?
I have often wondered why trainers don't teach to verbally challenge BG while shooting.
I don't know where you have trained, but several of the gun schools/instructors where I have attended do stress verbal challenges. The problems of verbal challenges is that they may sacrifice your element of surprise. Verbal challenges may also reflect to the shooter your unwillingness to shoot, opting to verbally challenge instead of engage with lethal force (as with McKown), and as such, the verbal challenge may work very negatively for you.
On the good side, a verbal challenge is good for summoning help, helps to show that you are the intended victim and not the aggressor, and it has the potential to actually work. A verbal challenge may be all that you have to offer to instigate a defense of the shooter is positioned such that you cannot safely engage him without harming others so you challenge the shooter verbally to drop the weapon, surrender, etc. while you move into a position for a clear line of shooting, hoping that your verbal challenge will be enough to keep him from completing whatever shooting task he is about to complete. So verbal challenges certainly have some tactical benefits in the right situations, but I would not worry making the verbal challenge out of fear that some lawyer may portray me in a negative manner for not making it while defending my life or the lives of others.