Who is "trained in safety" by your definition? What do you suggest?
By my definition? Back up chief. People are claiming that Zambrana had "safety training" as a result of having a CCW and/or from being a security guard. The question that should be being asked is what is the safety "training" that Zambrana had. Unless he is a firearms instuctor, chances are that his safety "training" was extremely limited to being told the basic gun safety rules, maybe having to identify them on a test, and then not being spotted violating them while otherwise qualifying. Chances are that during the course of his security guard and CCW instruction, he received no materials safey and handling instruction. He probably was not familiar with exactly what would and would not constitute an appropriate backstop to stop a projectile fired from his gun. He probably has never seen or learned the contents of a materials safety and handling sheet for his ammunition or chemicals used in maintaining his gun. Chances are, Zambrana through his CCW or security guard training never received instruction and became qualified to be a range safety officer for the gun range where he shots or for the security company for which he had worked.
His "training" was likely the same 5 or 10 minute guns rules safety review we have all had countless times and then some sort of correction, if needed, if he was seen to violate any of the safety rules whilst on the range. That isn't really "training." It is actually just a comprehension of minimal standards, nothing more. My 76 year old mother got all that "training" last year when she renewed her CHL here in Texas. It isn't training. It is just a brief familiarization, nothing more.
There are people in numerous fields who do undergo extensive safety training for materials handling. There are college degrees in occupation safety and some of which allow for specializations such as chemical handling, fire safety, etc. A lot of your disaster preparation and response training includes extensive safety training. Most of us don't get anything like that.
Both statements directly assert that because it was negligent and/or a violation of The Rules, it is somehow NOT "an accident".
If the gun fires because the trigger was depressed by the person handling the gun, then it wasn't an accident, except to say that it was unintentional. If the handler depressed the trigger and the gun fired, then there is nothing about the firing that is accidental. All parts worked as designed. That is exactly what is supposed to happen when you pull the trigger on a gun with a chambered round. That makes it a negligent discharge. It might be a negligent discharge that was accidental in the sense that nobody intended a round to be fired, but because due diligence was not observed, a round did fire. That is a negligent discharge.
That the gun was not oriented in a safe direction with a proper backstop when it discharged would appear to be another act of negligence. I know - it was an accident that the bullet shot through the closet wall. Wait, the closet wall isn't an appropriate backstop. That sounds like more negligent handling. It was an accident that Hannah Kelley was hit in the head. Well, obviously Zambrana doesn't have X-ray vision and so could not have been aiming through the wall to shoot the intended buyer's girlfriend, but Zambrana didn't have a proper backstop and did not know what was beyond his backstop which turned out to be another human. Negligence.
People are quick to say that "accidents happen" as if there is no blame. Amazingly, far few "accidents" happen when people pay attention to the rules.