From what I read, he was on the ground, still grappling with the guy. He reached for his tazer, and pulled out his pistol instead, and shot the guy.
Now, by coincidence, at our city's 4th of July fireworks display, just a week ago or so, something caught my eye. And, after I read about this incident, I can certainly see how this could happen. Our local police, carry .45 Sigs, and tazers. On a right handed cop I seen where the tazer was directly behind the pistol, and the grip, even thou it was yellow, had the same shape as the Sig. In a situation like what the officer was in, I see where the officer reached for the tazer, and mistakenly withdrew his pistol instead.
I know if it was me, I'd probably move the tazer over to a cross draw holster instead, just so in a life or death situation like this, I wouldn't pull out the tazer, when I needed to use the pistol. gypsyman