The problem with tasers is that they're single-shot weapons, at least the ones I'm familiar with. Miss, or find that the recipient is resistant due to emotional or chemical factors, and it doesn't work out.
Also, no LEO is going to pull a taser in response to an apparently armed adversary. You're going to pull your standard service sidearm, and make sure your opponent is not capable of further resistance.
What I don't get is why the training says to empty your magazine once lethal force needs to be used. Maybe this made sense in the days of six-shot revolvers, but given that police often use double-stack automatics, it seems like overkill. No pun intended. And it would seemingly leave the officers with nothing left to confront another threat should one manifest itself. Why not snap off a few rounds and re-assess whether the target requires more shooting?
The video of the ESB shooting last week was especially telling in just how much NYPD errs on the side of safety for themselves at the expense of their environment. They pumped the guy full of lead, and he was very clearly unconscious, but you can see on the video that they approached him as if he was a lurking crocodile, ready to spring back at them. This in spite of the fact that he never pulled the trigger at the police, though he did appear to brandish his weapon at them.
Again, I'm not blaming the individuals, but I do question the appropriateness of their training.