The employer created the opportunity for the bully to attack. So, the employer is responsible for solving the problem. One remarkable federal arbitrator cited the violence policy to hold accountable a supervisor and his manager for bullying. Fact finding is difficult because witnesses will not talk for fear of retaliation. Invent a method that guarantees safety for witnesses.
Bullies are repeat offenders.
The pattern of abuse is evident to all HR departments. There is some guilt implied because of this pattern. Take into account the perpetrator's record. Incidents are not unique and isolated. Risk management has data about litigation or settlement costs associated with repeat offenders.