Yes Nim wants to win but her actions are not without reasons. Nim has been hurt by her father's lackluster attitude towards her and the mom. I'm guessing her mom has been in that vegetive state for awhile now. That flashback shows that no one really cares for Nim. Aim's mom even said to the dad "we are suppose to cheer for Aim" or something along that line. Nim heard it when she was waiting for them to congratulate her when they saw her award on the table. She then saw the grandparents and dad taking pictures with Aim and her tennis trophy. She was very hurt and upset so she crumpled up her award. The only person that showed her genuine love besides her mom is Aim but Nim hates Aim for always receiving all the love and attention from everyone else. Nim's ex-boyfriend(Min) has always liked her sister and is now currently helping her out. Max is adamant about staying with his wife even and has told her point blank multiple time he's choosing Aim and staying with her. Preuk looked down on her during the meeting saying he only wanted a meeting with Aim not the assistant. Even the MIL despises her. I get where Nim is coming from. She has no love except for Aim's and her mom's.Nim just want to win...she wants to be better then her.... she had love...her mother's love...
The small flashback we got doesn't show her being tortured or bullied
But apparently her hatred for her dad, Aim's mom, and Aim is misguided.
My theory is
Nim's mom knew she's sick and wouldn't be able to take care of Nim. So she lied to Aim's dad saying that Nim is his daughter. I guess Aim's dad did cheat on her mom for him to believe that possibility. She probably thought Nim would be well taken care of because even though they aren't hi-so, Aim's family is quite well off. BUT THIS IS ALL PURELY CONJECTURE THOUGH.