Two prominent reason stand out--aside from his obvious ambition and lack of proper moral feeling--when contemplating Balram's motivations and justifications. The first reason by which he justifies his action is what might be called pride in having the nerve to do it. He explains to Jiabao that the nightmares that follow such an action are not those shown in Hindu movies, which start out with images and signs of some of the "36,000,004 gods,” but are of a different sort, at least for Balram. His nightmares, as he explains it, are such that he dreams of having lost his nerve; of having let Ashok get away; of having not fulfilled his ambition to get ahead in Indian life.
The real nightmare is the other kind. You toss in bed dreaming that you haven't done it--that you lost your nerve and let Mr. Ashok get away--that you're still the servant of another man ....
This ties in with the second reason by which he justifies what he did. He explains to Jiabao that what he wanted was a simple thing, a natural thing: He says he only wanted to be a man. To his way of thinking, in India, with its strangling caste system, the one way to have a chance was murder.
All I wanted was the chance to be a man--and for that, one murder was enough.
Though he says, "one murder was enough," he is inferring equally by this statement that the inverse is also true: One murder was requisite (i.e., necessary). It is by these reasons that he justifies what he did, regardless of the risk to his family: to have the nerve to act and force his way out of servitude; to take a chance to be a man.