Marius- a former royalist and Grandson of Guillenormand, leaves his home.
He had just witnessed his fathers death (Marius did not know his father). His father was a devout Revolutionist, and Marius decided to change his way's to his fathers ways.
Guillenormand could not stand his grandson's change (as he himself was a complete royalist) and so kicked him out of the house.
Marius also learned that his father was saved in battle by a comrade. Marius says to himself that he is indebted to his father's saviour. The comrades name: Thenardier