Daniel killing Eli symbolizes the end of a curse placed by a devil.
Daniel may act like he has the moral high ground by the very fact that he's not a religious zealot but he really is a thief and all of his awful actions make him a much worse person. The fact he's a murderer at the end after trying to break a man shows that he's only motivated by malice.
At the end, Eli tries to get Daniel to stop attacking him by screaming "We're family! We're brothers!", presumably a reference at least in part to the fact that H.W has married Eli's sister Mary. Except even if Daniel hadn't disowned H.W (and, well, wasn't almost certainly going to kill Eli anyway), this wouldn't have worked anyway, as Eli is utterly unaware that H.W was never Daniel's son to begin with, and so Daniel has never been a part of his family.
Eli's brother, Paul, only shows up briefly as the guy who reveals the prospect of oil underneath his family's property in exchange for $500 upfront, and after skipping town is never seen again. Unlike his brother, however, there are various subtle hints suggesting that he left a better impression on Daniel such that he may not be lying about paying Paul $10,000 at some point.
Paul is shown to be calm and courteous where it mattered, but also has enough self-respect to stand his ground when he feels he's being taken around in circles.
While he doesn't sugar-coat his spiels and lays out what he wants on the table, Paul also remains patient and respectful of his host. He also takes time to acknowledge and smile at H.W, something that Eli never really does.
The land's fate years down the line is sealed the moment Paul describes and points out where the saltwater from the wells is coming from. An experienced oilman like Daniel would almost immediately recognize that as a strong indication of salt domes and petroleum traps.
Lastly, upon shaking Daniel's hand and receiving a thinly-veiled threat if he was found to be lying, Paul doesn't flinch and remains calm, reinforcing his honesty. No wonder even a misanthrope like Daniel seems to respect him more than he ever does Eli.
Fridge Horror
Due to Eli "blessing" Daniel's oil rig and him abusing Daniel during the so-called "baptism", the curse was placed on Daniel two times.
