The theme of Battle Royale by Nyanya Li

Battle Royale is the movie of one such class-battle. Beginning with 42 students, the novel follows their fates as they battle each other. Among the special rules of the game is that someone has to get killed every 24 hours or else everybody gets killed, and that more and more parts of the island where they are playing become off-limits as the game progresses.

A very evident theme in Battle Royale is making choices for those people who you love.  
One of the main themes running throughout the film, is personal survival and the abandonment of notions of friendship, and all forms of socially learned behavior.
The aspect of sacrifice was also noted between the three protagonists, Shuya and Noriko, who ally themselves with Shogo.
Battle Royale shows us not only the love and relationships but also expose the ugliness of human nature. The students tried to believe in their classmates, but the game forced them to murder others, friends turn on friends. In order to live by their survival instinct, all the students started to kill others. They killed themself or other students. 
Although the environments were slightly hard, some people were still as kind as before. One of the protagonists- Shogo, who had a big advantage over all the other players: he's played this game before. He used his brain and skills to be safe, but finally in order to help Shuya and Noriko, he sacrifice himself.
Still, Battle Royale is a perfectly fine thriller, with a fun premise, quite well drawn-out.