What is written in the notebook is strongly implied to be a list of things that Tsukasa can use to blackmail other people.

Tsukasa is yandere because of her sister. Her sister is a giant airhead (strongly implied to be somewhat mentally retarded) and thus received special attention from her family. Also because of her condition, Tsukasa becomes the prime candidate to carry on the family's company (apparently her family is rich or something) so from young she has been saddled with a lot of expectations by her dad. The stress of this resulted in her yandere side.