If you were using a Taser as designed, a less lethal alternative to a firearm, then you could theoretically still pull the wrong one, shoot the suspect, and still be not guilty. For example, crazy man threatening police officers with a knife. Without a Taser the guy is subject to being justifiably shot. If a Taser or something like a beanbag shotgun or pepperball gun is on scene and there is time; an officer can make a go at less lethal in order to try to avoid killing crazy person.
Now, back in reality land, Tasers seem to be universally employed in the US as pain compliance weapons to induce cooperation in situations where lethal force would not be authorized. While it may be generally less damaging than the old maglite to the base of the skull trick the end result as we see here can also be fatal. Every time a taser is used as a pain compliance tool and someone is seriously hurt I hope the user goes to jail. In short, this guy should have never been reaching for a gun or a gun shaped taser.
I think the proper place for a Taser in a continuum of force is above OC or other irritants, just below baton strikes to the head, neck, or joints, and handgun.