There is something wrong that the video seems obviously exculpatory to the police and obviously damning to so many who watch it."
Those with professional knowledge can speak to it better than me, but I suspect the protocol in Britain would be to park at a relatively safe distance, order civilians to get back, call for back-up and specialist assistance, while monitoring to ensure that Mr. Powell poses no threat to himself or anybody else. What caused the situation to escalate to the point that the police felt so threatened that they needed to open fire on a mentally ill man carrying a *knife at his side was the arrival of the police. There is a serious problem in how US police perceive and deal with "threats". Mentally-ill people, even ones with knives, are primarily a threat to themselves.
[UPDATE: The Police stated Kajieme Powell charged at them and he was carrying a steak knife. It was being reported in Twitter last night that it was a table or butter knife).
I know that American police face different risks from British ones, and that gun violence is higher and police face a higher threat, so let's park the gun issue and look at the threat from knives on its own. In 2013 armed police were deployed in the UK about 12,000 times. They fired 3 shots and killed nobody. I don't know how many of those incidents involved knives, but I suspect it was more than one. The St. Louis PD bested that in 15 seconds when they fired 9 bullets into Mr. Powell. American gun enthusiasts and police officers always say "you don't shoot to wound, you shoot to neutralize the threat". The news for them is so do British police, and they successfully neutralize the threat with both fewer shots fired and fewer dead citizens. "But the British armed police are top marksmen!" is usually another reply. Well, if you can't see that that's an argument for better firearms training of US officers instead of an excuse for their poor accuracy, I don't know what's going to make you see.
The most disturbing aspect of this for me is that the police fired several bullets into Kajieme Powell's body while he lay wounded on the ground, and yet they apparently wanted this video released as it was "exculpatory".
There exists a very deep chasm between what the Police view as justified and what, I think, most reasonable citizens would. In a democratic country where the rule of law exists in such a difference of opinion the difference must always be settled on the side of the people, who are sovereign. In the United States it seems to be settled far too frequently, to put it at its lowest, on the side of the Police. That the officers involved in this don't see that this footage could be viewed in a different light says that the problem is a deep-seated problem with the recruitment, training and education of police officers.
All this makes me continually grateful and forever respectful for the British and Irish tradition of unarmed community policing.