Even if there were two different moves that officers needed to be trained in regarding the discharge of a projectile tazer and a firearm, you completely neglect that in a stressful situation (and don't tell me it wasn't stressful, any time tensions get high regardless of reason, adrenaline, cortisol, and a bunch of rabroad
her hormones start pumping in response creating stress) the human brain will fall back on what it knows to react to the situation most appropriately. If he had grabbed his gun instead of the tazer, his brain would have responded by utilizing parts that contained all the training and muscle memory he had when holding/firing a gun, resulting in appropriate discharge of the weapon.
Is this precisely what happened? Could this have been an actual act of murder? I don't know, I wasn't there and don't really care to dig and find out. But you discounting human behavior under stress doesn't make it what you want it to be.