I took Plainview to be an evil guy who would likely become uber-wealthy and a bit vindictive. As I said on my blog, my lack of faith in humanity might have tainted my viewing. I never saw Plainview as ever having a chance of becoming decent member of society. From the start, he was going to do whatever it took to conquer.

EDIT: Also, he showed signs of losing it or focusing too much on just winning very early on IMO. I saw most everything as a power struggle (telling the dad not to beat the girl, telling Standard Oil not to tell him how to raise his son, etc.) He would sell out everything, even his own pride (at the Church) if it meant getting what he needed.

Again, I saw it more as a study of a given path more than some sort of transformation from good to bad.