Two preppy college guys terrorize cottage owners on a lakefront,killing 1st the dog,then the little boy,the husband and then the wife is dumped from a boat into the lake... all for "fun".

Funny Games U.S. (http://www.imdb.com/title/tt0808279/) (2007) or the original Funny Games (http://www.imdb.com/title/tt0119167/)