The way I am thinking about the motive (to the extent that people even really need a great motive to kill) is (IMO):
- The pressure from the boat crash cases was mounting. Tinsley, the Beach family's attorney, had told him that he was going to have to pay a lot of money to settle the case. Alex told Tinsley that he was broke. Tinsley didn't believe it based on what he knew of Alex's caseload and was going to force Alex to turn over financial information. If his financial information came to light, it was going to become clear that he was stealing from clients. That would have gotten him disbarred and would have stopped him from making a legitimate living. Worse, it was going to turn off the spigot on his illegitimate income. Even worse, Alex would probably have to go to prison.
- If the boat crash case were to settle -- or go to trial -- he was going to lose everything, including Maggie's beloved beach house and Moselle, which was Paul's "absolute passion"
- Paul's criminal defense was costing a fortune and would have been making its way through the courts for years, which caused more financial pressure.
- Alex's usual modus operandi of tampering with the jury was not going to fly in the boat crash cases, because Tinsley told him that if he had even a hint of a whiff of tampering, Tinsley was going to immediately file new suits in Beaufort County -- where the Murdaugh family name isn't as powerful and Alex wouldn't be able to jury tamper -- naming Buster and Maggie. More financial pressure.
- Maggie was encouraging Alex to settle the suits, but Alex was backed into a corner and couldn't without all of the above consequences, which she did not know of.
- IMO, Maggie and Paul were becoming aware of financial issues through declined credit cards and bounced checks. Maybe Paul ("Little Detective") had even done some snooping and had unearthed some evidence of the ill-gotten gains.
- Maggie and Little Detective were putting pressure on him by finding drugs and confronting him. They were coming between him and opioids.
- IMO, Alex was having an affair and he also had to worry about that coming to light, because if Maggie divorced him, he would be financially ruined (I am basing my opinion about the possibility of an affair on Mushelle Smith's testimony that he had only been by at night to visit his mother a couple times in the years that she worked for Miss Libby coupled with testimony by Marian about how much time he spent there and Maggie's text saying that she was worried about Alex because he wasn't getting enough sleep due to all the time he was spending with his parents at night.)
- Alex's father, who was his protector and fixer for his entire life, was dying. There would be no daddy to help him soon.
- The above consequences -- becoming penniless and being found to be a thief and a liar and an addict -- would have ruined his social standing as a powerful Murdaugh.
Killing Maggie and Paul would remove several pressure points. The criminal boat case would be dismissed. Juries would be less likely to award big damages against him and maybe Tinsley would even drop the case. He wouldn't have to worry about letting down Maggie and Paul by being found out to be a con man and a thief and by losing Moselle and the beach house and the rest of the lifestyle they had become accustomed to. He would no longer have to worry about hiding things from Maggie and Little Detective. Maybe he would be able to find a way to replace some of the missing money before anyone became any the wiser, or at least before anyone made public accusations, enabling him to avoid prison.
Do I think these are great reasons for murder? No, of course not, but it doesn't matter what I think is a good reason -- these were good reasons to Alex. People have murdered for much, much less.