PTSD doesn't necessarily result in violent tendencies, folks really need to stop perpetuating stigmas and making generalizations like that.
I myself have suffered from PTSD for decades and spent the better part of a decade so absolutely terrified of getting seriously injured or seeing someone else seriously injured that I literally went out of my way to avoid physical confrontation as well as mitigate the risk of personal injury.
Heck, I didn't even own a firearm of my own until 2015, and to this day the mere thought of seeing real blood and gore ever again is enough to get my pulse racing.
Sure, I may be a bit emotionally unstable and quick to anger, but I'm not really a violent person, much less a homicidal maniac. It's just the nature of a survivor, that's all, especially ones who received combat training. How the heck do folks expect a warrior to react to a threat? Gardner was a soldier, trained for battle, a man who valiantly served his country, only to have a bunch of selfish, entitled, socially-indoctrinated nihilists threaten his property. How was he supposed to react? What would have been the "acceptable" course of action for him to take? I'm sure that all the cowardly, bleeding-heart pacifists would say that he should have just walked away, let them loot, pillage, and destroy his property, complete with lofty expectations of how the saintly insurance companies would totally pay for it all to be rebuilt from the ground up, completely refurnished, with every little thing accounted for, but that isn't how real life works. Assuming his business even was well insured, I doubt that they would cover the damages, and even if they did, they'd be looking to cut corners everywhere they could. Besides, what about sentimental value? That's not something you can cover or replace, but obviously wealthy, sheltered folks cannot understand nor appreciate sentiment because everything in their world is consumed and replaced in short order. For all the talk that gets thrown around about acceptance, understanding and equality, folks sure do go out of their way to look at others in the worst, most incriminating way possible.