Henry Dunn is killing everyone on the island along with Wakefield who is indeed still alive and it turns out that he's Henry's father. Henry is trying to win back the love of Abby and he had to marry Trish in order to bring Abby back to Harper's Island. Abby is his sister and he wants to keep her on the island with him like they talked about as kids.
Abby manages to escape Henry's clutches and kills him on the beach to save Jimmy and stop the murder spree once and for all.
The only people to make it alive off the island are: Abby, Jimmy, Madison, and Shea.