What motivates Hally to insult Sam and spit in his face? He does not like Sam meddling in his family matters.
He wants more respect from Sam and Willie.
He is distressed upon hearing of his father’s return home.
He is angry that Sam and Willie are acting like children.

He does not like Sam meddling in his family matters.

Explanation:

Athol Fugard's coming of age one-act play Master Harold and the Boys revolves around the characters of a white man Harold and his older black men-servants Sam and Willie. Set in the apartheid South African tea room, the plot delves into the issue of family, race, loyalty, and anger.

Towards the end of the play, Harold learned of his cripple father's return from the hospital. This made him so worked up that he even began criticizing and bad-mouthing his own father, which Sam tries to stop. This already outburst of anger led Hally to continue, even going as far as reiterating his white skin advantage over the black slaves. And when Sam told him to stop his nonsense and take back his words, he even attacked the old man, telling him "You're only a servant in here" and "He's a white man and that's good enough for you". He also spat at Sam to prove his point, asking them to address him as Master Harold and to never interfere in their family life, for they are just servants employed to help in the work and nothing else.