I think its fine, as long as you don't use ANY media from the film, (clips and sound tracks) as this is all COPYRIGHTED!
The fact that your not going to be making money on it makes it all alright.
P.S. Stop asking questions and write some code! Finish the game (I know you want to, and Gary needs to do some work now that hes got his notebook) and then worry about copyright!
Cause the worst that can happen is you will need to change a few details. (Characters, Music) This will be easy compared to actualy makeing the game, writing the code, modeling the weapons and characters.
+ I want a copy A.S.A.P (as you know i have seen battle royale, a great film, now i want to play the game!!!!)
-There are 10 types of people in this world, those who understand binary and those who don't.
-Wheres the "ANY" key?