APaul Dano was originally cast as Paul Sunday only, while Kel O'Neill was originally cast as Eli Sunday. When O'Neill left, Dano was recast as both Paul and Eli because, at that point, it was too late to re-shoot scenes, so the film plays out that Paul is Eli's identical twin. Paul Dano stated to an NPR interviewer that he had less than a week to prepare for the role of Eli.
AIn the beach scene, Daniel makes reference to a memorable incident at the "Peachtree Dance" which they both supposedly attended, and he looks at Henry as if expecting an immediate response. Henry clearly doesn't recognize what Daniel is talking about, and Daniel begins to suspect that he is a fraud. He later interrogates Henry further before shooting him.
AIt's rather obvious that Daniel (Daniel Day-Lewis) and Eli have been feuding ever since Daniel snubbed Eli publicly at the dedication of the Little Boston well. Coupled with the money that Daniel never paid Eli, $5000 of an agreed-upon $10,000 total and that Eli was really a fake in his faith-healing -- Daniel roars at him about not being able to restore HW's hearing -- and that Eli's motivation in having Daniel be baptized was to humiliate him publicly, it's no wonder that Daniel cracks in the last scene and kills Eli.Additionally, the fact that H.W. Plainview (Russell Harvard) was now going to be a rival to Daniel caused more fury and anger in him, which Daniel projected onto Eli. When Eli visited and told Daniel about Mr. Bandy's land, Daniel had already drained the areas with the wells he'd drilled surrounding that area. Daniel wanted nothing to do with Eli. He killed Eli out of greed and hate. The last line "I'm finished!" meant he was finished with his guest along with his life. It could also mean that Daniel had eliminated his last possible source of competition, Eli.Another explanation lies in Daniel's view of family. He accepted H.W. as his son, as he had control over the child. As H.W. became autonomous, he withdrew from Daniel, destroying Daniel's attempt at family. Eli arrives, telling that he and Daniel are now brothers -- HW had married Eli's younger sister. As Daniel never sought this creation of family with Eli, he decided to destroy it. A popular explanation of Daniel's killing of Eli and the meaning of the film's final line is that throughout the movie, Daniel devolves into a purely evil human being. From the beginning where he is so poor that he is willing to crawl on his back for miles to claim a silver find to the end where he has become rich and is now almost completely insane, Daniel has descended into becoming truly demonic. Ultimately, he is so evil that the murder of Eli means absolutely nothing to him, and his final line, "I'm finished," cements the fact that he has "finished" his descent into evil. Another interpretation to the final line could be that Daniel meant that he was finished "eating". This could either mean finished eating the meal that can be seen when the two were talking, just before the murder or that he was done "eating" Eli as Daniel can be heard saying "I told you I would eat you up." while he chases Eli around the bowling lane.