There are no reported details of exactly how the police encountered the security guard.
And that is how EVERYTHING should be determined. Anytime a gun is involved, particularly when brandished... the chance of police shooting first and asking weapons later is... extreme. As in... it's going to happen, guaranteed. Any other result is a fluke and you know it. How police entered that room, and what they saw determines right from wrong.
Did the security guard have time to react, and what was his reaction? Hell, when police storm into a building I doubt the good person with the gun has any time at all to react in a way that can save them from holding a weapon. There are countless cases where flinching or having normal human reactions gets people killed.
It's because of the guns. Because of how guns force us to react and fear for our lives. We throw police into those situations, standard policy forces those deadly confrontations where things DO go horribly wrong. Because we're making split second decisions to pull a trigger that ends someone's life. Under threat of them doing the same to you. It is an impossible, extreme stress, scenario. We are only human. We will never get that right at all times, hell, count yourself luck if you get it right half the time.
It's the setup of the scenario that fails us. We need different / stronger policy on guns in this country. We need different / safer police protocols, but that's only possible if we find a way to keep people safer. Both public and officers involved.