In the movie Dream House, near the end its revealed that:
Its actually the man-next-door (Ann's husband and Chloe's father) who kills the Ward's wife and two daughters. We are shown that he is entering the house carrying two guns and shoots the daughters with them. We are also shown that he tries to kill Ann (his wife) to get the insurance on Chloe.
So why did he try to kill is neighbors? It would have been no use to him (he only would have had to kill his wife to get the insurance). What did he expect from killing the neighbors?