Well, obviously the story was made so that EVERYONE could see the film. The book is a "young adult" book I believe. So converting it to a movie has to even lessen some of the details so that those parents that bring kids to such movies can be saved from all the gory details... even though everything is pretty straight forward. I was surprised and satisfied with the way the opening scene was during the games. It wasn't as brutal (probably should've been) as I thought it could be but it definitely showed enough. Obviously many young kids got iced because they couldn't stand a chance to the older, more developed kids who wielded sabres and such. I thought it was portrayed very well.
To clarify, I still enjoyed the movie, I just wished they would have stayed a bit closer to the source material. I understand things have to be cut/shortened for time considerations, but it was more than that. Sanitizing the movie for the kids is silly when most of those same kids read the books.
As for battle royal, I thought it was well known that The Hunger Games idea was based off it? I mean Battle Royal was another one of these books that came out in the 90's that dealt with a lot of the same issues... but more gory in detail.
Is it? I'm not a superfan (have only read the first book so far) so I don't know much about it. If that's true, then my apologies to PA (though I think calling it "a ripoff" is still a bit much)