Vendettamorphosis
Vendettamorphosis (noun) - the process of transforming from a mild-mannered, peace-loving individual into a revenge-seeking, grudge-holding fiend. It typically occurs after someone has been wronged, and they become fixated on seeking retribution against their perceived oppressor. Symptoms may include obsessively plotting schemes, muttering curses under one's breath, and an intense desire for justice (or revenge, depending on your perspective). Treatment may include meditation, therapy, or simply learning to let it go.
"After years of putting up with his boss's abuse, John experienced a vendettamorphosis and decided to get even by stealing all of his office supplies and leaving anonymous complaints to HR."