No, in my opinion Balram is not justified in killing Ashok. There is no doubt that Ashok is a "bad guy." He's an abusive drunk through much of the story. He's an incredibly selfish individual as well. He's wealthy enough to have two private drivers, but when Ashok takes Balram to Dehli, Balram is forced to live in a cockroach infested apartment, while Ashok and his wife live in splendor.
At one point in the story, Ashok beats Balram for taking his wife to the airport without his knowledge. Ashok also likely has ties to organized crime which is indicated by the bag of money he's always carrying around to use to bribe public officials. Ashok is a selfish and corrupt businessman more or less.
I might feel differently about Balram's justification of killing Ashok, if Balram had done it out of self defense or had done it to protect someone else. I just don't get that feeling. Balram lured Ashok from the car and killed him with a bottle. Ashok then took the money and used it to set up his own business AND begin bribing officials to ensure the success of his new business. I don't see Balram being much different from Ashok. It's simply a substitution of one corrupt businessman for another.