you realize everything about that 20-foot rule is assuming the cops don't already have weapons drawn and trained on the guy? in this case they were already aiming at him and he wasn't nearly close enough for them not to have time to squeeze a trigger. he wasn't charging at them either. if they want to use lethal force they should wait until he's actually close enough to do anything. "this guy has a knife and is kind of walking in our direction, lets shoot" is bad