Bumping this topic:
I read the book a month ago or so and watched the film recently. I've heard some opinions about Eli being manipulative and the "ultimate villain" from people who have seen the film and some scenes may suggest so, but having read the book, I believe that the relationship between Oskar and Eli is genuine friendship. In the book, Eli even states that it's been a long time since he/she last had a real friend.
Even in the book, I think that the fact that Eli is male is hardly a central point to the story. The story has a lot of elements though, moral aspects (killing people to survive), being an outsider, friendship etc. The open ending is very interesting, leaving the audience to have their own interpretation.
I personally believe the ending to be an hopeful one, although lots of shit happens during the movie.
I recommend reading the book if you like horrorish type of stories. The movie is pretty good too.