He was not directly defending his father, but rather believing that he was protecting his daughter, Felix, from harm by the monster. De Lacey, blind and old, was entirely dependent on his children, Felix, Agatha, and Safie, for his well-being.
When the monster, who had previously shown kindness to De Lacey and his family, was unjustly accused of stealing and attacked by the villagers, De Lacey, believing the monster was a threat to his children, defended him. This act ultimately led to the monster's further alienation and despair.