There are many kinds of violence in the workplace. Some very subtle and covert, but just as harmful if not more so, than overt violence. It's up to the employer to pay attention to an employee's attitude and approach. And watch for any signs that there can be trouble brewing. And even if they do, it can be only temporarily.
Many employers are anxious about the aspects of liability.
And bad publicity if they take a more active role when there's violence in the workplace. It will help greatly if an employer gets some kind of specific training in dealing with violence in the workplace, particularly in preventive measures. If an employer learns to recognize at least some of the signs that a person is possible of violence, they can head off trouble before it starts by simply not hiring them. Sometimes this doesn't work and an employer is faced with an employee who is possibly violent. Then the best this employer can do is try to keep everybody calm and stop the situation from getting any worse.