In The White Tiger, is Balram justified in killing Ashok?
Certainly, murder is never justified. As the other educator pointed out, Balram did not kill as an act of self-defense, nor did he kill Ashok to protect anyone. For his part, Balram was solely motivated by his desire for revenge.
Having said this, Ashok was no angel. In fact, his behavior was often repulsive and his actions abhorrent. It was Ashok who let Balram take the blame for Pinky Madam's actions. In the story, Pinky Madam ran over a child while driving intoxicated. Instead of stopping to see to the plight of the child, Ashok conspired with Balram to leave the scene of the accident. When Pinky Madam protested, Ashok gagged his wife to keep her quiet.
At their home, Ashok dragged Pinky Madam upstairs, with a scarf still tied to her mouth. Later, Ashok made Balram sign a statement identifying him as the sole party responsible for killing the child. Despite his seething anger, Balram had to keep his emotions under check. He maintained that the jails of Delhi were full of drivers behind bars because they were "taking the blame for their good, solid middle-class masters." A poor man could leave the villages, but the masters of the Coop still owned them "body, soul, and arse."
Balram was further incensed by the fact that his own grandmother supported the prospect of his incarceration. In India, it is a matter of pride for impoverished families to support the sacrifices their sons and daughters make for their masters. Even the judges take bribes for choosing to ignore the real facts about each case. The one left holding the bag, of course, is the driver himself. This state of affairs is what leads Balram to contemplate killing Ashok (and to finally go through with it).
Since men like Balram have no recourse to justice, they resort to administering their own brand of equity. Balram's actions (and also Ashok's) demonstrate the impact of corruption on both the rich and poor.