Assuming this actually happened, there is something that was really never made clear in that thread that could have an impact on how things went down, at least in my opinion.
Was this an actual "Security Guard" or a "Loss Prevention Officer"? Security Guards are normally uniformed employees of an agency other than the retailer to whom they are assigned, and SHOULD have more thorough training than Loss Prevention Officers who are, as a rule, plain clothes store employees whose job is to look for shoplifters.
One might conclude that the reaction to a uniformed individual might be expected to be somewhat more restrained than the reaction to a non-uniformed person.
Bottom Line is this.....no matter what the security person's motivations, his tactics and execution sucked. I do have a certain amount of sympathy for him, however, since, in addition to his physical injuries and the medical bills arising from them, he is no doubt unemployed since his employer will drop him like a used Trojan just to cover their ass financially. On the other hand, stupidity is often painful, so.............................
For future reference for the security person:
1) No matter how scary the big, bad gun might look, so long as it remains holstered it is NOT a threat.
2) Keep the armed individual in sight while calling 911, and wait for the professionals to arrive and assess the situation.
3) Do not approach the individual or TAKE ANY PHYSICAL ACTION unless or until the weapon is drawn from it's holster. Then, and only then, can a REASONABLE person be expected to believe that the armed individual is up to no good.
YMMV