I agree it's bizarre, I agree that it's possible that he was planning a murder/suicide and chickened out, but I disagree that Alex is highly intelligent. I think part of what makes the crime so difficult to understand is that it was really stupidly carried out. And I think the people who think there is more to the story are giving Alex too much credit. He's not playing three-dimensional chess, he is a good ole' boy whose family has been in control of the county for a hundred years, whose relationships with local law enforcement are very chummy, whose family has always gotten away with everything -- and he thought he could get away with it because of the cover afforded him by the family name, power, and money.
Things were coming to a head, his financial crimes would be exposed if the boat crash case went forward. Paul's criminal defense (with the same lawyers now defending Alex) was going to cost a fortune and leave him with nothing. Mark Tinsley, the Beachs' attorney, is a formidable adversary and Alex had recently been disabused of the notion that Tinsley would throw the case because they had a history. Mark was going to bankrupt him and expose his financial crimes, because the Beach family wasn't satisified with an insurance settlement -- they wanted the financial settlement to hurt and they were not going to be satisfied with a little hush money. Alex would lose Moselle and the beach house and have to face Maggie's feelings over that. He would be humiliated and end up in prison. The only way to stop things, at least for the time being, would be to have a death in the family that would buy him time to come up with a plan, and bring both sympathy and the dismissal of the boat case.
For what it's worth, I think he would take back Paul's murder if he could. I don't think it occurred to him that he would not be able to hide the financial crimes until he figured out some way to replenish the money or cover his tracks more thoroughly; now he's killed his son for nothing.
