Who is Bukong and why does he treat Nick the way he does?

Bukong is a mahout. This is the name given to those men who work and care for the elephants on the Hawk's Nest plantation. There is an air of resentment among the mahouts towards their British colonial overlords. Indeed, many of the mahouts openly support the Japanese in their invasion of Burma—present-day Myanmar—as they believe this will hasten the establishment of Burmese independence from the British.
After the Japanese take over, they commandeer the Hawk's Nest plantation to use as their local headquarters. This is where Nick is forced with others to work long, hard hours in the stifling tropical heat. Bukong has been put in charge of the work detail. He is a cruel, vindictive man who sadistically enjoys beating the children under his watch with a cane, especially young Nick. He singles out Nick because, to Bukong, he represents the hated British colonialists, one of whom is Nick's father, currently interred in a POW camp.
Bukong has years of pent-up anger and humiliation inside from performing dangerous work for people who don't respect him or the other mahouts. And as all that seething rage finally comes to a boil, he takes out his anger mainly on Nick, flesh and blood of the hated oppressor.