When someone is poised to claim what the character wants most—a promotion, a lover's attention, the respect of peers, etc.—sabotage might seem like the best way to win. But the unintended consequences, guilt of underhanded tactics, and fear of exposure can transform what could have been a victory into a hollow achievement or well-deserved failure.

Regrets, Guilt, or Shame (primary); Dark or Disturbing

PLOTTING REVENGE, RUINING SOMEONE'S REPUTATION

BEING JUDGED, BEING LABELED, BEING UNABLE TO ACHIEVE A DREAM, BETRAYAL, COMPETITION, CONFLICT, CRITICISM, FAILURE, HUMILIATION, ISOLATION, LEAVING NO LEGACY, LETTING OTHERS DOWN, LOSING FINANCIAL SECURITY, LOSING ONE'S SOCIAL STANDING, LOSING THE RESPECT OF OTHERS, MEDIOCRITY, NEVER FINDING HAPPINESS, NOT FITTING IN, REGRET, REJECTION

Rationalizing their actions, thereby stunting their self-awareness and personal growth
Becoming hardened to the rival's suffering (and, therefore, the pain of others)
Squandering energy on sabotage instead of building toward success and the future
Alienating allies or potential supporters with their obsessive focus on sabotage
Thwarting someone who could have been a valuable partner or ally
Developing a reputation for dishonesty or ruthlessness
Feeling compelled to continue undermining others to maintain their position
Losing sight of their original goals 
Turning an apathetic rival into a vengeful enemy the character must deal with

Acting overly friendly or helpful toward the rival to mask their true intentions
Seemingly avoiding direct competition or confrontation 
Expressing feigned surprise or sympathy when the rival experiences setbacks
Praising the rival as being more talented or better so the character won't look like a rival
Using covert methods to enact their sabotage (laying traps in secret, covering evidence of tampering, etc.