Why would you start to enjoy 'giving him a taste of his own medicine'?

When the abuse has been "gratuitous" (sadistic), repeated, and protracted, conceptual closure is not enough. Retribution is called for, an element of vengeance, of restorative justice and a restored balance. Recuperation hinges on punishing the delinquent and merciless party. The penal intervention of the Law is often therapeutic to the abused. Regrettably, the victim's understandable emotions often lead to abusive (and illegal) acts. Many of the tormented stalk their erstwhile abusers and take the law into their own hands. Abuse tends to breed abuse all around, in both prey and predator.