His fears are stoked by his mother, who scolds him constantly and compares him unfavorably to his smarter, better-behaved younger brother Sonny.
Larry resents his mother, who constantly pushes him to excel at school and frets aloud that he will amount to nothing, like his drunken father.
On Christmas morning, Larry wakes up early and finds that Santa Claus has left only a book in his stocking, while there's nothing but a toy gun in Sonny's.
In that instant, Larry realizes for the first time that there is no Santa Claus, only an impoverished mother who's been striving vainly to raise a decent son under miserable circumstances.
By shunning the book and stealing the toy gun, Larry has broken his mother's heart, and convinced her that he will become a rotter like his father.