It was a powerful and resonant movieall the way up to the end which was just sort of “WTF!?” Was there some thematic way the very last scene was supposed to tie in with the rest of the movie that doesn’t involve the use of a sledgehammer and mind altering drugs?
I don’t know that there’s any official answer. I don’t think that P.T. Anderson has given any kind of dfinitive explanation, so mine is only my own interpretation, but here goes.
Plainview kills Eli because Eli is the only person who can see into his soul, just as he is the only one who can see into Eli’s. Each is able to identify the other’s deepest shame – the deepest source of guilt – and force them to confess. Plainview’s ego won’t let him forgive or forget Eli making him confess that he’d abandoned his son. Even after he is able to extract Eli’s confession that he is a hypocrite and a fraud, it’s not good enough. Hypocritical and fraudulent, Eli may be, but he still had Plainview’s number about HW, and Plainview knows it. He can’t abide that judgement. It feels like the judgement of God to him, and even forcing Eli to say that God is a superstition does not assuage his guilt or sense of being judged. He thinks that by killing Eli, he will kill Eli’s judgement. He will erase his own self-indictment and guilt.
Whether it works or not, is a little unclear, but I think the fact that there is a witness to his murder shows that his judgement and guilt is still there, and is now magnified exponentially. His final line, “I’m finished,” could be taken as a reference to his own life.
To wank things even further, I think that allegorically, the movie is about the struggle between the two most powerful formative American influences – capitalism and religion. Both have creative power, both are essential components of the American mythos, but both are also prone to corruption. Anderson is showing religion (or at least self-appointed religious authority. I think Anderson is distinguishing that from faith per se) as hypocritical, dishonest and ultimately self-interested. Capitalism is ultimately rapacious, amoral, sociopathic.
Anderson shows capitalism and religion as being at war with each other. Capitalism appears to win, in that it gets religion to sell out (and symbolically “kills” its authority)…but then there’s still that witness…still a moral judge.
That’s more or less how I read the movie, and I take it even a step further, in that Plainview (no accident that name) was giving his own twisted sermon to Eli, hoping to convert him to his own amoral view.
That’s pretty much what I got out of it, too, which didn’t seem anywhere near as deep a message as most people thought it was.
Plainview killed his ersatz brother Henry. He also banished HW, his adopted son. When he kills the only man who has ever forced him to confess this, Eli, he succeeds in killing superego, thus divorcing himself complete from humanity, finishing what his greed started. And I agree that it is an allegory about the evils of capitalism, seeing all religion and all family as lies.
The fact that Henry and HW are not blood relatives is not irrelevant, it illustrates that all family are in fact not the same human beings as we are, but we have a real connection with them nonetheless. Henry was a fraud, but Daniel suspected it from the get go (and HW knew). Despite suspecting Henry of being a fraud, Daniel at first accepts him, and then on finding out the truth kills him. In all fairness, Daniel is a fraud too: he is not an oil man originally, he got lucky; he is not HW’s father and uses that relationship to swindle people out of their oil rich lands pretending to care about families.
Ultimately, after HW leaves and Daniel repudiates his relationship with the only human being he ever deeply loved over a long term, the path of the confrontation with Eli is set.