Dear Health,
The question of rodef begins when the drunkard is about to drive. When the cop was on Floyd’s back and gasping for breath, even had he been a full rodef before (He was not one bit a rodef to begin with.) he regained his status as a life worth protecting. Stopping a rodef, is not a license to kill.
PS The minchas chinuch mentioned above can be found at 337 4. Please take a look!