When I first saw the footage of George Floyd being asphyxiated by a policeman’s knee on his throat, my reaction was pretty standard. My eyes bugged. I stood up. I exclaimed something like: ‘Bloody hell!’ We’ve all seen the video dozens of times now, but it’s worth clinging to that initial shock, the better to appreciate that America’s spontaneous collective revulsion in response to such grotesque abuse of power was genuinely commendable.