Whether or not the 17-year-old victim was charging officers with a knife matters only as to whether the police were trying to act appropriately or not (i.e., Were they trying to use what they thought was a safe alternative to their gun or were they trying to punish someone for misbehaving?). The knife doesn't matter, even a little, when it comes to this sequence: suspect is tased, police immediately see the need to attempt resuscitation, suspect dies.
That sequence obviously has nothing to do with knives. The knife is a red herring that distracts from the issue of a taser not being as safe as its manufacturer claims.
I read an editorial yesterday (it's seriously not worth the link-love) that essentially argues that if someone dies after police tasing, the death is only the outcome of their choice to gamble with the law; if you mess with the law, you deserve whatever you get. It's a naive opinion that's out of touch the realities of what happens in spaces other than those occupied by comfortably plump opinion makers.
The idea that suspects deserve whatever they get and its companion opinion that we must support our boys in blue under any and all circumstances have several frightening features. They accept the police using scrutiny-free deadly force in any situation. They stand on the footing that the police can never be wrong. They don't care about the hundreds of deaths that have followed taser use. They don't care about cops tasing children, people running, mentally ill people, or people with increased sensitivities to tasers (which could be a large population indeed). They especially don't care about tasers being used disproportionately in minority neighborhoods. They are opinions based in the luxury of never having had an encounter with a bad cop (and yes Virginia, they do exist).
No matter how much you love tasers and the police, and are rubbed the wrong way by people calling for their examination, there is no sturdy moral defense for giving cops a weapon that randomly dispenses an in-the-street death sentence.
So yeah, it's not the knife. As with all of the incidents it's: suspect tased, suspect dies.