[obviousagent]

Ok, I will try to answer the big question from an armed Police point of view, I admit it is a UK cops point of view but from what I know use of force is similar across the pond.
There are several laws and legislations governing the use of force and paperwork will always follow any use of force. Use of force is a scale with tactical communication( that's talking) at the bottom and lethal force at the top. It is interesting that in the UK lethal force does not actually have to be capable of killing, it represents a "high" level of force; i.e. a baton strike to the kidney area or the groin can be classed as lethal force.
Now in the UK only specially trained officers carry firearms routinely and their use is very limited, usually requiring higher authorisation. However officers can use the firearms without this if certain criteria are met.
As in all cases though the decision to pull the trigger lays with the officer, it's their shot to justify.
So has anyone heard of perceptional distortion?
You can experience a traumatic/stressful episode as a group and not one recollection will be the same. Everyone will see things differently. Also this entire conflict could have lasted seconds, things can slow down under stress they can speed up, the slightest movement can seem massive. You get tunnel vision, that famous pinpointing of focus. He may not have seen the other officer going for his taser, he may not have known the background to the incident, he may have thought or seen something the other officers were not aware of, things could have gone so quickly he may have used his firearm the same time the officer used his taser as he saw things differently. I know from experience The officer involved shooting will have to justify his actions to all and sundry.
I truly hope that what has been said by the family of the youngster is wrong and is grief and wanting to apportion blame, but the bottom line is they called Police for a reason, a reason that had an episode that can give the human body strength and aggression beyond the norm, also that reason had a weapon.
If he is found to be in the wrong then he should face the full power of the law in your country; if not it is something that will stay with him for the rest of his days.
I hope I don't come across as being one sided that wasn't and isn't my intention, just trying to illuminate from the blue side of the house.