Gentlemen, let me preface this with two things; first, I don't think Ofc Potter should be culpable for murder. I think it truly was an accident. She most definitely should be terminated for the mistake. Second, as a former training officer, former tactical operator, and (very unfortunately) as an officer who has had to use lethal force, I can tell you from experience that once an adrenaline dump has occurred, any and all common sense, fine motor skils, and cognitive reasoning / ability goes out the window. So do most of your five basic senses.
Muscle memory and training immediately kick in and she instinctively went for her primary instead of the Taser like she had apparently intended. From the video, you can hear her yell, "Taser Taser Taser!!!" She then squeezed the trigger one time which is how a Taser is deployed. There is no recoil, no automatic reload of the darts. A second deployment of the Taser would require a change of cartridge or what is called a drive stun (the pressing of the nose of the Taser directly to the skin or surface of a combatant). Her fine motor skills were gone and she did not have the cognitive ability to differentiate between the primary and the Taser. This is where the adrenaline dump took over.
It mattered not that the Taser was yellow or weighed about 7oz. and her Glock would have weighed around 1.5 to 2lbs and was all black. Your 5 senses at this point are diminished and she would not have been able to tell the weight difference or seen the color due to intense tunnel vision. The gun could have weighed 50lbs and she would have never felt it. Same reason you hear about people lifting cars or ripping doors off hinges, the super adrenaline dump heightens other things.
The adrenaline dump is so intense and so quick that it is over within a few seconds. The officer was coming down from her dump and that is how she realized she actually shot the subject with a lethal firearm and not a non-lethal Taser.