Why does Marius leave the home of his grandfather and refuse money from him?
When Marius was a baby, his grandfather, Monsieur Gillenormand, demanded the child from George Pontmercy, his own son-in-law, under threat of disinheriting the child. Gillenormand, whose daughter died in childbirth, had no use for the "blockhead" (162) or his differing political opinions. Of course, Marius grows up not knowing this particular situation and feeling as if his father is somehow undesirable.
As an adult, Marius learns that his father had been coming to mass just to catch a glimpse of his son and that he had "sacrificed himself that his son might someday be rich and happy" (168). When Marius learns his father had earned the title of Baron and that he, himself, had inherited it, Gillenormand becomes extremely angry, arguing that Marius' father was a "bandit" (178) and a "traitor." Though his title came with no money, Marius, out of anger and indignation at the lies he endured and the lost time with his own father, leaves his grandfather's home, refusing to take any financial help.
Hugo, Victor. Les Miserables. Trans. Charles Wilbour. New York: Fawcett, 1961.