It’s the driving philosophy behind all the “unarmed” BS. There is no such thing as fair play in combat, it is no kind of game at all!
I should have been more clear about the disparity of force concept.
In self defense cases, disparity of force means that your assailant(s) do not necessarily have to have a weapon to be a deadly threat.
Thus, if you are attacked by a bigger man (like Mike Brown) or by a group, or by someone much younger and more agile, or are knocked down by surprise, you can still consider them a deadly threat and use deadly force against them, and be justified.
I can see how you could take it the opposite way, which is what happened in England. There they say that you may not use any more force than your attacker.
The law does not work that way here, or at least it is not supposed to.