The diary is key to understanding Heathcliff's character. His childhood was somewhat horrendous. He was an orphan living on the streets when Mr. Earnshaw brings him to live at Wuthering Heights. Although Heathcliff and Catherine get along, Hindley is very jealous and when his father dies, forces Heathcliff to live like a servant and he gets no education. Hindley's abuse gets even worse after his wife dies and Heathcliff's only friend is Catherine. Even Catherine betrays him when she says she could never marry Heathcliff because of his lack of social standing. The diary helps explains much of the adult Heathcliff's lack of feeling towards others and his search for revenge.
Catherine's diary entry illustrates a castrated Heathcliff that you can say was bullied, humiliated, and shunned from society. In the diary entry you can tell that the only ones who really saw Heathcliff for who he was, kind and caring, was Mr. Earnshaw and Catherine. Mr. Earnshaw trusted him so much to the pot where Harley was so jealous that when Mr. Earshaw died he took any status and future education that was promised to Heathcliff. In this diary entry you can really see the injustice that was done to Heathcliff just for him existing. And seeing the injustice almost makes the reader justify why Heathcliff does what he does and become what he is.
In Ch.3 Lockwood the primary narrator in "Wuthering Heights" is forced to spend the night at the elder Catherine's room in Wuthering Heights because of stormy weather. He spends a restless and sleepless night and to while away the time he begins skimming through the books he discovered by chance on the window ledge of the room. The elder Catherine had "covered every morsel of blank that the printer had left" of all these books with her own commentaries. "Some were detached sentences; other parts took the form of a regular diary, scrawled in an unformed, childish hand."
From these "diary" entries we learn of the unhappy childhood of Heathcliff: "Hindley's conduct to Heathcliff is atrocious." Now that Mr.Earnshaw is dead and Hindley has become the master of Wuthering Heights, he begins to abuse and ill treat Heathcliff. Catherine records that even Frances, Hindley's wife joins in hurting Heathcliff: "Frances pulled his hair heartily." To make matters worse Joseph the puritan servant, constantly sermonises to Heathcliff and prevents him from enjoying his childhood. Joseph is instrumental in doing his utmost to separate him from Catherine his soulmate and only source of comfort and joy in this cheerless and miserable place. Once he seaprates them when "they had made ourselves as snug as our means allowed in the arch of the dresser." Heathcliff and Catherine rebel, but they are punished severely by Hindley.