Although the information in this post was accurate based on reporting available when it was made, new information available demonstrates that much of it is actually incorrect.
I'm very interested if there's friendly fire casualties.
Possible, however this article quotes the Sheriff as saying that the two deceased persons were shot by the "initial shooter".
https://heavy.com/news/joshua-jamal-williams/
"We had one initial shooter at the scene, who hit two victims inside the location. Both of them have been pronounced deceased here at this location."
The article indicates that besides the "initial shooter" and his two victims who are both deceased there were also two others shot who are currently in stable condition.
Good Ol' Boy's explanation goes along with the articles I have been able to find:
1. Joshua Jamal Williams walked in with his brother, apparently intending to use the shooting range. He was carrying an unholstered pistol with an extended magazine. It seems clear from the way events unfolded that the pistol was not just unholstered/uncased, but it was also loaded. One photograph of the crime scene shows a Glock with an extended magazine lying on an asphalt surface
2. Noah Fischbach (a clerk at the store) told Williams that it was not allowed to carry a pistol in that manner.
3. Williams apparently took offense and fired a "warning shot" then shot Fischbach, who died.
4. Williams killed a second person (Veronica Billot, 59), described as "a customer", who also expired. It's not clear at this time if she was killed while Williams was in the store or after he exited. Her body appears to be near the door in the photos of the crime scene, but that doesn't mean that's where she was when she was shot.)
5. Williams exited the store and was engaged by "several individuals" and was shot and killed. It is not clear at this time if he was engaged before or after leaving the store or both.
6. Two other persons were shot and are hospitalized in stable condition. At this time there is no information on whether those persons were shot by Williams or hit by "friendly fire". It does appear from the Sheriff's statement that these two persons were shot outside the store.
Not much information here to go on in terms of anything to be learned. The only thing I can come up with without speculating is this.
Don't assume that a person will react the way you expect them to react. We don't know if Fischbach was obsequiously polite or imperious when he told Williams about the rules, but it is pretty clear that he was not prepared for the reaction he got.