Chibiusa hated you and Kousagi because you guys usually get your parents' attention instead of her, she was an attention seeker, and she enjoyed being the only child.

Chibiusa did everything she could to get Usagi or Mamoru to hate you guys, but no matter what her attempt was, it failed.

That was when she went extreme.

During math class, you and Kousagi were doing multiplications, and Chibiusa decided that she was determined to get the attention she deserves.