soberups
Pees in the brown Koolaid
Cops have rules of engagement when using deadly force. Your scenario probably does not apply to the officer.
Cops are human beings with survival instincts, just like civilians.
From a legal standpoint they, like civilians, are allowed to use a reasonable level of force to meet a threat.
A reasonable person could draw the conclusion that a 300 lb man who assaults you, overpowers you and attempts to take your gun away is doing so because he intends to use it on you, and if you are outweighed by 100 lbs and are in imminent danger of having your gun taken away then your only reasonable option at that point is to kill him before he kills you.