I think that the notebook was brainwashing them into having no creativity so the notebook could control all of them. During the first part, the notebook showed them creativity, but when the yellow-ish guy painted a picture of a clown, the notebook poured black paint/ink all over it, suggesting that the notebook wants control of the creativity. The notebook slowly made them insane, making them have twisted and dark types of creativity (the raw flesh cake, glitter on the heart, etc.) so they thought creativity is bad. Then the notebook said that they should agree never to be creative again, suggesting that they should end creativity so the notebook could control them.