The Wrath of Vajra is a Chinese 2013 martial arts action movie presented on DVD.
Kill or be killed.
In 1930s Japan, a death cult working with the Imperial Army buys children and trains them as deadly assassins. When 8-year-old Vajrasattva accidentally kills his own brother in a fight for food, he makes a secret oath to take revenge. As an adult, he emerges as the cult's best assassin, returning to China where he joins the Southern Shaolin Temple to protect his people from Japanese invaders. When the death cult opens a branch in China and starts buying local children, he sets out to save them and avenge his brother. But first he must defeat their three greatest warriors in a battle to the death.