Daniel was not "evil" or a sociopath
This is just my personal opinion from my observations after seeing the film many many times.
Yes, he "used" his child as a sweet face for his business but he truly cared for him and it shows several times throughout the movie. It was horrible that he abandoned him, but he didn't do it because he had no use for him after he went deaf. He did it because he didn't know how to tend to a child who was setting fires to his home and he didn't understand his condition or how to deal with it. Again, he felt horrible about it and after being forced to confront himself with it, he brought his son back. It's not like he just put him on a train and wished him luck, he had him sent to a home with other kids. It was a terrible thing, yes, but the fact that he realize that it was terrible and that he had made a mistake shows that he DID have compassion for the boy. Most men, especially single men, probably were not good at raising children on their own at the time. Not to mention juggling the several oil wells and businesses, I can imagine it was extremely difficult. But he did, for years, when he didn't have to.
While murder is never justified, honestly look at the people who he did kill. The man pretending to be his brother was taking advantage of him, and it hurt Daniel to know he was still alone in his mindset of just wanting to be alone and enjoy his success. He was lied to, manipulated and fooled into believing he had family when he really had none. The man was using him. He didn't deserve to die, sure, but he was no innocent. As for Eli, well just watch the movie and you can identify the reason he had for murdering him. Not to mention he had reached a point in his life where he was all alone, depressed and an alcoholic. Eli, after humiliating him and always trying to win one over on him, was trying to take advantage of this and just like the man who pretended to be his brother, was only wanting a slice of what he worked so hard to acheive for years. If both of these people had stayed out of his life, they would have survived, but they were absolutely no better than Daniel. If anything, they were worse because they didn't want to work for their success, they wanted to leech off of Daniel's.
The final scene with his son, he said some awful awful things, but it was because he felt betrayed. All Daniel had ever known was his business and his son, and to have his own son attempt to compete with him (as he saw it) as well as marrying the daughter of his enemy and running off, was obviously very hurtful for him even if in a way, he did deserve it. In turn, he wanted to hurt his son emotionally in the way he was hurt. The flashback scene shown shortly after was obviously written to show he really did love his son and this was what pushed him so deep into depression that he was hardly recognizable anymore.
In the beginning of the film, we see that he turns a place that was basically a slum into a nice place to live, providing food, roads, schools and even the church despite hating religion. He mourned the man who died on his oil rig and instead of just cracking the whip and forcing everyone to continue working, he allowed them a day or two to mourn as well. People claim he took advantage of Bandy but in reality, he just outsmarted him and did nothing illegal. Same with Eli. He was an aggressive business man, just like the hundreds that exist today that operate the businesses you probably support on a daily basis.
Was he perfect? No. Was he a saint? Hell no. Was he a psychotic, sinister sociopath who went around stealing and murdering for the hell of it? No. He said himself, he wanted to be left alone and to be away from people. If the man posing as his brother had never attempted to take advantage of him, if Eli hadn't attempted to revel in his pathetic state while trying to get money from him, he would have never killed anyone. It's not really an excuse, but to me, the comeuppance Daniel gets at the end is what he deserved and earned through his mistakes and actions through the film. Even with his enemies dead, he still has to live out the rest of his life with his worst enemy, himself.
Let me know if I forgot anything. I also want to hear your opinion on these things.