With all due respect, that's not a "universal rule".
In my Village (in IL, but 'red' DuPage County) we are allowed to use deadly force to, "protect life and property".
Our Village's firearms laws consist of one half of one type written page (unlike Chicago's 69 pages) and I reread it over and over again to make sure I was reading it correctly, but that's it, "life and property".
Since the laws regarding murder, assault etc. are State laws the local ordinance will not give you cover for blowing scum away.