There are slight variations on this part of the story, depending on which version you watch. In the original series, Barnabas had already become a vampire and was going to make Josette one too so they could spend eternity together. While on Widow's hill, Josette said no to this plan. They fought and Josette fell off the cliff, leaving Barnabas responsible for her death.

Then Barnabas's father had him chained up and said he had gone to England. He didn't want his family associated with death by plague. I am not sure why this was a consideration, but this is the plot line.