After posting this article, I find the most plausible motive is that he and an accomplice shot his wife and son away to silence them. Discovery would have blown up his whole life. I think drug dealing and money laundering with the cartels might be involved.
His world was coming undone and he was about to be found out. He acted out of desperation. Even the attorney for the deceased girl admitted that had the murders been an act of vigilantism as Murdaugh contends, his clients would have settled with the previous offer of insurance money and dropped the punitive action (the Murdaugh parents regularly supplied alcohol to the minors and fostered an atmosphere of binge drinking and partying - and it was all posted on the internet) because they wouldn't have wanted to do that to a man whose family was murdered.