Not going to respond to the ancilary situations that some others brought up as they have zero bearing on the original situation.
The guy was obviously coherant, able to put sentences together and well aware of the situation. Given that situation there is no reason to believer he was anything near or resembling helpless or incapable of using the knives. Having one in each hand kind of gives an inkling as to his probable intent. One knife, there may have been some doubt, two indicates he is prepared and intends to use them.
The tape (first youtube link only as I don't trust anything from huff post to be unedited for pure bias intent) is incomplete and we were not given the original call. I don't know if anyone else had been threatened prior to PD arrival.
First off let me state something. There is no requirement that an Officer subject themselves to death or injury prior to taking action. Anyone who thinks there is some kind of tv-land requirement to let the suspect actually attempt to inflict death or harm first is being unrealistic. I have some other words but that would cross skuzzy's line. There is a definite line between an acceptable risk and being stupid in taking a risk. Going up and trying to remove knives from an individual who is prepared, with two out in hand and ready to use them is not taking a risk, it is being beyond stupid. Anyone who thinks it isn't shouldn't be out unsupervised or around moving machinery.
This individual had plenty of opportunity to drop his weapons and comply. He did not. After the flash bang he moves to intercept the dog with a knife before the handler pulled the dog back. That confirmed intent to harm. He removed other options at that point. He was on higher ground in terrain that precludes a quick and precise movement to contact with him to try and have a physical removal of the knives. I would have preferred a taser be used first given the situation however the PD may not have that option. A taser is not universally issued nor approved for use by all law enforcement agencies. Had he been tased and dropped to the ground he could have been instructed, again, to drop the knives then cuffed, had he complied. Had he not complied then the only other option was still deadly force. Tasers are also not a fail proof method of gaining compliance. They are usually effective but not always and anyone who trusts it to work 100% of the time will be disapointed and maybe end up dead.
Anyone who wants to have someone with a knife or knives taken out, hand to hand in a similar situation is welcome to do so on their own. Responding units will be there later to call the morgue truck for you should you fail or an ambulance if you are lucky. That type of action is neither advised nor required by any law enforcement agency or military unit in combat. It makes for great tv and movie scripts where they use rubber knives and everyone follows a script but not in real life. One should never confuse the two.