I think it was obvious that this killing was a response to a loved one whose medical claim was denied, and they died as a result. For it has all the earmarks of a revenge killing, to me at least.
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Sound to me like your assessment is the correct one. Scratching the words on the cartridges tells me the guy was seething with anger over a medial claim denial that lead to the death of a loved one. The guy is likely former/present military/law enforcement by the way he handles and clears the weapon.
he was going to testify on something..so maybe someone didn’t want to get nailed by him.