He is driven. He doesn't delve into the origins of the drive, though he is duped to some extent into familial sentimentality by an imposter who claimed to be his "brother from another mother." In a moment of weakness, Plainview decides to believe the lie in order to assuage a part of himself that he hates. This is the sentimental aspect of the human personality. Long buried, Plainview opts to ressurect what little sentiment lay at his foundation. He errs in so doing and kills his faux sibiling as punishment for the internal humiliation that accompanies all victims of fraud. The murder is a foregoine conclusion once Plainview discovered the fraudulent scheme. It is justified.
Daniel Plainview followed his own drive and kept his own counsel. Yet, the arrival of H.W. Plainview, likewise a fraud constructed by Daniel Plainview, revealed sentiment. Plainview held out the boy as his own purely for econmic gain by skillfully placing the boy adjacent to him during town meetings gathered to either approve or deny mining privileges. He played upon the sentiment of others. Random justice would play upon his own sentiment by the introduction of his "brother" into his life, and the injuries suffered by H.W. while playing at the site of Plainview's first gusher. H.W. lost his hearing and was left to observe this world by the acts of its inhabitats instead of the hollow words that emanated from their mouths. His first subject would be Daniel Plainview.
Daniel Plainview represents the story of man in terms of his base arrival into the realm of life, without regard to "superstition," which is a word he used to describe the belief in God. He was thrown into this crowded life and was left with the choice of following his own internal engines or those that society had constructed for those who might be less in tune with their plight. Plainview is skeletal man. His bones are his soul. He manipulates thier movement for his own gain, which is driven by his need to make as much money as he can in order to escape human society. In the end he realizes this goal, thus realizing his own misery -- a misery that was kept at bay while he was in his productive era.
The only suspicious aspect of Plainview's character was his reclusion into his mansion and solitude toward the end of the film. I suspect that, had Plainview acutally lived, he would have worked until the day he gleefully died.
