If he'd let williams walk up to him and then open the knife, it would be too late to stop him.
Not sure what's your stake in this, but you keep quoting the 21 ft rule and how that if the guy rush the cop etc etc...
Yet you keep ignoring the fact that the knife was closed, and the guy had his back to the cop up till the point he was shot.
I sure hope you are not/will never be in a position of authority, consider you can justify shooting someone in the back who is walking away and unaware of you and killing him, just because he has a closed knife in possession.