In "Wuthering Heights" what does Catherine's diary reveal about Heathcliff as a boy?

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.