One would have to show that the employer knew of a specific threat, or even a generalized threat, and failed to take any reasonable measures to prevent it. For instance, maybe there is a series of violent crimes in the parking lot at night and the company fails to install lights, fails to hire security, etc. AND prohibits employees from CCW. OR another example, let's say the company has an employee that has been vocal about making verbal and/or physical threats and the company is derelict in taking any action against the employee and prohibits employees from CCW... In both examples say the worst happens. Yes, I think the company could be held liable in court, after the plaintiff victim employee spends thousands of dollars and years duking it out in court.
I personally won a case where a business owned a parking lot that was notorious for property crime and the company took no measures to secure the lot. They were on notice and didn't take reasonable measurs, so my client won after his property was damaged and stolen. Not a perfect metaphor but close.