The desire for revenge is a dark secret that has the advantage of being relatable, because who among us hasn't been wronged and wanted to get back at the person who did it? 

Dark or Disturbing (primary), Betrayals and Violation of Trust, Illegal or Immoral Acts

FANTASIZING ABOUT KILLING SOMEONE, MONITORING SOMEONE WITHOUT THEIR KNOWLEDGE, RUINING SOMEONE'S REPUTATION, SABOTAGING A RIVAL

BEING PITIED, BEING TAKEN ADVANTAGE OF, CHANGE, COMPETITION, FAILURE, HUMILIATION, LOSING ONE'S SOCIAL STANDING, LOSING THE RESPECT OF OTHERS, NEVER FINDING HAPPINESS, NOT BEING IN CONTROL, TRUSTING OTHERS

Being unable to think about anything other than their desire for revenge
Seeing the target through a skewed lens; not seeing them clearly
Their negative worldview seeping into other aspects of their life
Their blind pursuit of justice making them reckless about their own safety 
Lost faith in the legal system making them doubtful of other institutions
Crossing moral lines to get their revenge
Becoming overly sensitive to any perceived insult or slight
Believing they'll never find happiness or peace without getting back at their enemy

Pretending they're still friends with the person they're plotting against 
Speaking highly of the person to others
Never contacting or speaking to the intended victim
Traveling long distances to meet with associates in places where no one knows the character 
Carefully surveilling the enemy to learn about their routine and habits
Subtly gathering information that will help them carry out their revenge
Planning their revenge alone (to avoid the risk of being found out)