Kids kill other kids in lord of the flies, so yeah its like BR. The Running Man you can make it similar because the lead character is being excuted in a 'stage-like' setting and is treated like he is on a show. Battle Royal has the same effect, in the beginning of the movie we see camera's everywhere all round a bloodied girl who just smiles. So, I take that the people either like it, or don't like it but will continue to support it anyways.
Point being, just because kids are killing kids, doesn't mean it's being "ripped off". It's just life, authors, film makers, and even just artists are going to draw on another previous artist's work and make something different. Do THG and Battle Royal share common elements? Of course, kids fight to the death by killing each other. But the context that leads to that plot are vastly different in each story that saying one ripped off the other is silly. For, Battle Royal was okay (film wise, I have been wanting to test out the book). But it left me underwhelmed simply because the back story to it wasn't developed, in that you didn't know 'why' or the exact reason why Japan was doing this other than that they felt they had to suppress their citizens for some reason. BR can by the bloody, more violent story it wants, but I feel THG is the better told story.