Moral dilemmas and temptations, duty and responsibility, relationship friction, failures and mistakes, ego-related conflict

The character's outer motivation is everything, and their number one goal is to reach that milestone. This need can be so great that when someone or something comes along and threatens that objective, the character may be tempted to do whatever it takes to win. The act of sabotaging someone can generate inherent conflict, but the prior mental struggle, the Do-I-Or-Don't-I? inner turmoil, can pose intriguing questions and add rich layers to a scene.

The character spiking a rival's drink or food prior to an interview or big game
A politician digging up dirt on an opponent
Entrapping a romantic rival to set them up and gain "evidence" against them
Giving someone the wrong answers to an important exam
Holding back useful information
Badmouthing an ex to win a child over to one's side
Leaking an unflattering story about the rival to the press
Knowingly sharing information about a rival that can be used against them
Creating an emergency for the rival's family to distract him or remove him from the competition
Hiring someone to physically take out the competition

Trying to make plans without getting caught
The character having to be subversive and sly when they're not good at it
Trouble finding dirt on a rival
The character having to cozy up to people they're just using as a source of information
Time and energy being diverted from the character perfecting their skills to sabotaging a rival
Having to hire shady individuals to do the character's dirty work
Financial expenses (to hire a private detective, pay off informants, etc.) 
Increased self-doubt, since the character knows they can't beat the opponent through fair means
Interacting with the competitor on a daily basis and having to act like everything is normal