A lot of people don't like this scene because it's overwrought, although I disagree with that assessment because I have witnessed people (both in person and via video) undergoing psychoses and emotional breakdowns who acted in a similar manner to this. It has been years since I last watched the whole movie, but it brought up something that has been on my mind lately. This movie seems to be about ruthlessness or American cynicism or capitalism, and that's probably what its makers intended, but to me, it looks like an indictment of the optimistic modernism of the early 20th century. This is not incompatible with (what I assume are) its intended themes.
In a nutshell: there was an optimism in the United States in the early 20th century that we'd reached the end of history, and There Will Be Blood showed the final outcome of that optimism. Let me explain.
Whenever I read contemporary American texts from the early 20th century, I can almost feel the smugness radiating off the screen, or page, as the case may have it. People then were convinced that they had found The One Way Forward for humanity, that a benign liberal humanism would create a perfect world. There was a lot of Utopian thinking, a lot of faith in progress. The Theory of Evolution was still new and technology was advancing by leaps and bounds. Skyscrapers, a kind of symbol of the 20th century that set it apart from the 19th, were already being built in New York and Chicago. Thanks to technology, man was finally master of his own destiny.
One found this same optimism in Europe up until World War One; after World War One, the Europeans became disillusioned with modernity, T.S. Eliot wrote The Hollow Men and everybody realized that scientific progress was being used to make horrifying weapons and not Utopias. The Europeans saw their countries ravaged by the first World War, and the ironic pessimism of postmodernity was already in its nascent stages. In the United States, however, the first World War was something for returning veterans and newspapers, not a visceral event that everybody saw firsthand. There was no blood and thunder on the shores of the United States itself. Americans wouldn't see that firsthand until the 21st century when a couple of planes were flown into the World Trade Center, an event that would sound the death-knell for American optimism. We are still working through that process, but it has begun, although not everyone can see it yet.
There Will Be Blood is a lovely film because it shows the final outcome of this. The unfettered freedom of the individual, the secular liberalism, and the optimistic humanism all culminate in a charnel orgy where the most vicious person claws their way to the top. You can almost see the beast emerging from the optimism of modernity as Daniel Day-Lewis, a rich pragmatic oil tycoon, hobbles forward, face twisted in rage while pointing at himself and screaming "I am the Third Revelation!"
The really sad thing is that American capitalism was better than the alternatives. Neither European fascism nor Communism were able to provide better societies than American capitalism. In fact, they were even worse. All three were different attempts at creating a workable modern world. The aforementioned death of American optimism has led to an amusing phenomenon where ineffectual revolutionary movements have popped up, complete with clueless college students proclaiming the virtues of anarchism engaging in "anti-fascist" demonstrations. Sorry, guys, I think you got the wrong century. Nobody wants it.
The crossroads the world is coming to is the point where modernity itself is seen to be defective, and we realize that the whole modern project was unworkable to begin with. I do not know what will replace it, but we can't keep it up forever. The capitalistic American liberalism has only proven superior to Communism and fascism as a result of its ability to maintain a more open society and, more importantly, the fact that it has simply survived longer. Whether it's Josef Stalin smoking a Herzegovina cigarette while signing off on a list of gulag-bound political enemies, or Hitler cramming people into ovens, or an oil tycoon beating some poor loony preacher's head in with a bowling pin, modernity always seems to terminate the same way; the beast emerges and devours.