To add to what Sean said you could look at specifically who succeeds and who fails in getting their revenge. Maybe they all succeeded on a certain level and then failed on another level. When you examine each character you can ask yourself these questions:
1. What was this character's original plot for revenge?
2. Did this plot succeed? (Maybe it succeeded but did not bring about the results the character intended.)
3. Was the character satisfied?
4. Was the character justified in wanting revenge? (Also was the severity of the revenge plot appropriate for the crimes commited against the plotter?)
5. What did the character learn from the results of his/her revenge?
6. Why did the character succeed?
7. Why did the character fail?
These all deal with character growth which should be very important in your essay. How does revenge affect character growth?
Tyler