If he'd let williams walk up to him and then open the knife, it would be too late to stop him.

I notice that the inquest jury, unlike us, who have had a chance to see all the evidence; : were split on whether the cop was justified or not:

http://revolutionarypolitics.tv/video/viewVideo.php?video_id=13963

and that the police didn't have enough evidence to pursue criminal charges.

Sadly one can't 'assume' the review board had other evidence. Police forces throw their officers to the wolves if it's politically inconvenient.
qjs