The idiot...
had a round in the chamber.
When you load a clip into a Glock (or a typical automatic) the gun still will not fire. You have to "chamber a round" by usually pulling back on the firing mecanism. This puts a bullet in the chamber. After you have done that, if you pull the trigger the gun will fire.
So, the idiot does a number of things. He uses his own gun instead of a demo to demonstrate to school (or wherever it was) ant it has a round in the chamber. The only time a gun should be loaded is if it is in a secured safe, on your person, or in your hand ready to fire. If he was bringing it in for demonstration, it should not have been loaded. If he wanted to show it being loaded, there is no reason for a round to be chambered.
If you watch the video, he takes out the clip off camera, but the gun is "chambering", that is, pulled back to load a bullet. So the putz basically arms the gun. Then notice he releases it, which makes the gun ready to fire. Then he actually pulls the trigger. If he followed the rule that you don't pull out your duty gun unless you intend to shoot someone, this never would have happened.
I hope he loses his job. If you don't have any idea of firearm safety after being a police officer, you need to go. He is either insanely careless or stupid. He could have killed anyone in there. Then he has the nerve to pull out a rifle. I would have walked out.