I just watched the movie Eden Lake (2008). It has a very sad and disturbing ending. The bad guy who killed Steve and also burned Adam alive for his disobedience doesn't get any punishment but the lady who accidentally kills a small boy gets killed by Brett's family. What kind of an ending is that? What is the director trying to say to the audience from that ending?
Well, we can't expect every movie to have a happy ending. Sometimes film makers prefer to make movies in an unconventional way. Eden Lake is one of those movies happens to fall under that category with anticlimax, a disappointing ending for the audience, a good example for these type of movies in the recent times is Gone Girl (2014).
Here's what director, James Watkins says about the ending in an interview.
The most chilling thing about the film may be the ending. Did any of the producers pressure you to give the film an 'happy ending'?
The ending was discussed: so I came up with an even darker ending! You don't want to know…
and in another interview, he says,
Were there many discussions about the ending?
Yeah, lots of discussions. I didn’t want to have an ending… she’s a school teacher, she’s been through this horrible journey, she’s reached almost this point… you know when she gets the knife and it’s kind of almost pathetic when she hits the dad with the knife and she’s almost at the end of the road I think. I didn’t want her to turn into this take charge Ripley, Sarah Connor, pull a grenade out of her dress and kill everybody Death Wish.
From his words, I understand that he doesn't want to end the movie like the most of the slasher flicks that are made in mainstream cinema. But he wants to end it in a much more darker and realistic way.