One of the maid, Angelique Bouchard (Eva Green), in Barnabas' house fell in love with him, but he didn't return her love. Instead his love went to Josette. What Barnabas didn't know was that the Angelique was a witch. She killed Barnabas' parents and put a spell on Josette so she jumped from the Widow's Peak. Barnabas was sad, so he jumped too, but ... he didn't die. Apparently Angelique has turned him into a vampire.
Then Angelique told all the people in Maine that Barnabas was a vampire, and they locked him in a coffin and buried him.
