Although it probably makes the incident worse, not better, I would like to stress that this was not a rank amateur, 'riding shotgun', these reserve deputies go through intensive training, to the extent that they can perform certain duties (directing traffic, parking patrol) without oversight.
But there's the nub of the case. He was only authorized to use the Tazer (he may even have donated it to them); the pistol was his own. So it was "pay to play" in the sense that his donations got him off the boring duties the auxilliaries were brought in to fill and onto the exciting car chases.
I do not wish to disparage our PCSOs but I'm curious about how closely their role parallels that of reserve deputies or whatever this man's job title was.
This is only with the aim of putting it into a UK perspective. Our gun laws preclude a private weapon getting into a police stop situation.