I found this on www.stopcyberbullying.org and it's something that can be easily applied to situations among adults:
“The Vengeful Angel”
In this type of cyberbullying, the cyberbully doesn’t see themselves as a bully at all. They see themselves as righting wrongs, or protecting themselves or others from the “bad guy” they are now victimizing. This includes situations when the victim of cyberbullying or offline bullying retaliates and becomes a cyberbully themselves They may be angry at something the victim did and feel they are taking warranted revenge or teaching the other a lesson. The “Vengeful Angel” cyberbully often gets involved trying to protect a friend who is being bullied or cyberbullied. They generally work alone, but may share their activities and motives with their close friends and others they perceive as being victimized by the person they are cyberbullying.
Vengeful Angels need to know that no one should try and take justice into their own hands. They need to understand that few things are clear enough to understand, and that fighting bullying with more bullying only makes things worse. They need to see themselves as bullies, not the do-gooder they think they are. It also helps to address the reasons they lashed out in the first place. If they sense injustices, maybe there really are injustices. Instead of just blaming the Vengeful Angel, solutions here also require that the situation be reviewed to see what can be done to address the underlying problem. S there a place to report bullying or cyberbullying? Can that be done anonymously? Is there a peer counseling group that handles these matters? What about parents and school administrators. Do they ignore bullying when it occurs, or do they take it seriously? The more methods we can give these kinds of cyberbullies to use official channels to right wrongs, the less often they will try to take justice into their own hands.