Good idea, even if it is not a new film idea.

Unfortunately, very "cheap" implemented in many scenes.
Above all, the end is how far-fetched, as if to 3/4 of the film had no desire to turn.
The trainer of the small "serial killer" is represented to me a bit too weak and too often overwhelmed to go through as a true "professional".
Overall, the plot of the movie look after ten minutes is to see through and the end.
The interesting places ("the revenge / training") is a bit short and staged of course totally exaggerated.
Power simply to look no fun when a 12-year-old any beefcake beats that have a greater arm circumference than his head.