Not so, read this law: Conspiracy Against Rights
Now either "two or more persons" who "conspire to injure, oppress, threaten, or intimidate any person [...] in the free exercise or enjoyment of any right or privilege secured to him by the Constitution or laws of the United States, or because of his having so exercised the same" must be held to protect against just such scenario as you have posited, or there IS no rule of law.
Laughable rubbish. You have no “right” to a job, and your boss can fire you at any time for any reason or no reason at all, subject to the terms of the employment contract you both signed. He just can’t fire you for an illegal reason. This is well settled employment law in every at-will employment state. I’m sure this guy’s contract said that employment was conditional on him following the terms of the employee handbook, which forbade him from bringing a gun on property.