Always aim for the legs. What the forget.
Yeah that makes sense in your video games but that's not how real life works. A gun is an officer's last resort, if you are shooting someone as a police officer you are eliminating the threat, you are not disabling, you are not warning, you are using deadly force. Not only does shooting someone in the legs not guarantee threat elimination but, it is much harder to hit someone's legs opposed to the center mass. NOT ONLY THAT but, even if you shoot someone in the legs it isn't guaranteed that their wound won't be non-fatal. If you hit an artery and there's no ambulance on scene the wound is likely to be fatal.
The actions of the police officers are a tad bit excessive but justifiable. If the man in the video had a knife and was moving towards an officer with it, his intention was to kill the officer. The officers are supposed to respond accordingly.