When jurors reached a verdict this year in the trial of Alex Murdaugh, a fourth-generation South Carolina lawyer, they found him guilty of murdering his wife and younger son. But looming large over the trial was another tragic death: that of Mallory Beach, a 19-year-old woman who was killed in a boat crash involving the dead son years earlier.

The authorities said that the son, Paul Murdaugh, had been driving the boat when it crashed in 2019, and a lawsuit brought on behalf of Ms. Beach’s family — threatening to reveal the lies and thefts surrounding Alex Murdaugh’s finances — was part of the tightening circle of events that was closing in on Mr. Murdaugh in the weeks before the murders.

This week, Ms. Beach’s family agreed to settle that lawsuit with a co-defendant, the owners of a gas station convenience store that sold alcohol to an underage Paul Murdaugh — who was using his older brother’s ID — on the night of the crash. The $15 million settlement, which must still be approved by a judge, comes on the eve of a civil trial against Alex Murdaugh and the company, Parker’s Kitchen, that had been scheduled for next month.

P.K. Shere, a lawyer for Parker’s, said in a statement that the company had been at a disadvantage in facing a joint civil trial with Mr. Murdaugh.

“Given the outsized publicity this case has received, being tethered to a convicted murderer all but ensured Parker’s would not receive a fair trial,” Mr. Shere said. He added: “Our hearts continue to go out to the Beach family. We sincerely hope that all involved parties will find some measure of closure.”

The settlement brings to a close a case that had put a magnifying glass over the Murdaugh family and their influence in South Carolina’s Lowcountry region, years before they became the focus of documentaries, true crime podcasts and newspaper headlines. Mark B. Tinsley, the lawyer for Ms. Beach’s family, said the parents were satisfied with the payment, some of which will be used to fund an animal shelter in Ms. Beach’s name. He said they had also wanted to avoid a trial that would give Mr. Murdaugh, who had been scheduled to give a deposition in the case on Tuesday, a platform for his own version of events.

“He’s a broken person,” Mr. Tinsley said of Mr. Murdaugh. “And to give him a chance to spin whatever lie he’s going to tell next, they didn’t want to be a part of that.”

Alex Murdaugh had denied responsibility for the crash in court papers, but Mr. Tinsley had argued that he was partially liable because he “condoned, encouraged and facilitated” Paul Murdaugh’s underage drinking while giving him access to the family’s boat.

Ms. Beach, an animal lover and former high school soccer player, died when the boat she was on with friends struck part of a bridge. Mr. Murdaugh was charged with boating under the influence, but from the beginning there were questions about whether he was receiving favorable treatment from the authorities because of his family name.

He was still facing those charges over two years later when he and his mother, Maggie, were shot to death on the family’s rural hunting estate.

Jurors deliberated for less than three hours in March before finding Alex Murdaugh guilty of the murders, and a judge sentenced him to life in prison. He is also facing an assortment of financial charges, accused of stealing about $8.8 million in all, including from clients he represented who had been injured in vehicle accidents.

The Beach family’s lawsuit came up during the murder trial, when prosecutors said that Mr. Tinsley’s efforts to force Mr. Murdaugh to produce financial documents was one of two issues in the time before the murders that were threatening to expose his financial malfeasance. The other was a confrontation hours before the murders in which his law firm’s financial chief accused him of pocketing a six-figure payment that he was supposed to pass along to the firm.

Mr. Murdaugh admitted stealing funds from those around him but was adamant that he had not killed his wife or son. He said he believed his son was killed by someone angry about his role in the boat crash.