If the guys stealing the trucks were just independent hijackers selling the oil to his competitors, that had no reason to go to his house to try to rob or threaten him. Was the guy he chased into the woods just a regular cat burglar?
wondering about this myself as well as the would-be burglar at near the beginning. maybe the guy with the indoor tennis court? He told Abel he liked Abel but he wasn't happy that Abel had stolen some of his customers. That guy also implied he's not as nice of a guy that Abel thinks.
I don't think it was the guy Abel confronted at the barber shop - he didn't seem to have the balls for that kind of violent action, just cashing in on the stolen oil from Abel.
I like that they left those two things unresolved, not everything wrapped up in a neat little package for the audience like it usually is in movies.
Makes the violence scarier as well, I think that was what the movie was trying to portray - a sense that the violence was all around at the time, something you couldn't get away from, and could be both people you knew well or didn't know at all.
I believe it was the mob. The main mob guy comments on how that happened to him as a kid and he also remarks that Abel had moved in on their territory and that they had not done as much as they would normally have to stop it. That doesn't mean that they didn't try a few things or lower level mob was doing it.