I have NEVER heard that "flexing" meant someone was cocking back their arm to punch. I have spoken with many many people in a fight and not once had any of them ever used the term "flexing" to mean anything other than a reference to posturing. All of them were quite clear that the other guy was getting ready to punch them and they struck first.
I even "googled" to see if there was a definition I wasn't aware of for "flexing" and Urban Dictionary even defines that " to flex" essentially has one of two meanings. It can mean to
show off, to gloat, or to boast, which is the most popular definition of the word; however, it can also mean to put on a fake front, to fake it, or
force it. The second definition is usually used in conjunction with the first — as in, someone who's gloating about something that they've really got no right to gloat about, lying about an accomplishment, or exaggerating the truth.
You asked if a landed punch could result in death. That was the comment about you making a "red herring" argument. Schilling never once articulated anything about Balboa getting ready to punch him. It would have been quite easy for him if he really thought that and really though he was protecting himself. Schilling was looking for an excuse to pop the guy and he did (side note: not a discussion on whether or not Balboa got what he deserved, which I think he did with his background and other stuff). Schilling is a prime example of someone thinking they know what the law is and throwing out the "I was in fear for my life" phrase with no articulable justification on "why".
BUT, let's say I agree with you that Balboa was only drawing one arm back (which is contrary to the body posture of the video). You would STILL need to articulate what you saw prior to your actions to show that they were reasonable. Which Schilling didn't do. Again, we circle back to self-defense is an AFFIRMATIVE DEFENSE. It means that you admit you broke the law. It means you assaulted someone. But, you have to say exactly why you did so and that by doing so it was reasonable. Then a judge, jury or PA/DA looks at your reason(s) and determines whether it was justified and reasonable or not.