I tend to see that his dependency on MM and his reputation drove him. I think he believes he loved her but his definition of love is skewed.
With his father dying, the father's trust will kick in for all of the Murdaugh siblings. AM wanted cash more than anything. With the law firm on to him, cash was going to get tight and he was going to have to fork over everything he had with little hopes that the law firm would not report him to LE. MM had to have known they were in financial trouble. And, there were the financials that would need to be filed in the boat case. He can say what he wants but his crimes coming to light must have been terrifying for him. For a lawyer who had not committed crimes, it might have been a nothing burger but not a man who had stolen millions.
During the direct examination of him, AM says he was not worried about the theft discovery nor the financial materials needed for the civil case. Other lawyers confirm that the financial declaration was not a big thing. True. But, if you are AM, the financials and the other thefts of money you have done are putting you on a path to being discovered right quick because the lawyers for the plaintiffs are not going to accept that you have nothing and forensic accountants are going to be up in your business. IF PM is dead, so is the criminal case. IF PM is dead, it will be hard to get a judgment against AM because there is no interview/interrogatories/depositions of PM. IF MM is dead, he does not have to face the humiliation of being broke in front of her. IF MM is dead, she won't be able to have any interviews/interrogatories/depositions either. She can't answer what he did or PM did or if she had a call about him. The only way to stop the PM boat accident stuff is to kill them off. He will still have one son, his money, and his trust fund, even if his other financial crimes come to light. And, I think he believed the law firm would just let him pay everything back without going to LE. JMHO.