You should never respond in kind. If a terminated employee makes a threat of a lawsuit you should either not respond or make a neutral comment. Call a witness if an employee makes a violent threat. If you suspect that the employee is about to become violent and cause harm to others or company property, call security or the police immediately. In general, the less said to co-workers and other employees about an employee's termination the better.
People are naturally curious about what happened.
However, you do not want to infringe on the terminated employee's privacy or leave yourself open to legal action because of something you have said. If appropriate, you might want to review company policies, performance expectations, etc. with employees. You should, however, be prepared to inform those impacted by the employee's departure such as immediate co-workers, subordinates, clients, etc. Tell them that the employee no longer works for the company and how his or her duties and responsibilities will be carried out going forward.