“No one will ever forgive me.”

Those were the words Dahud Hanid Ortiz wrote in an email to an in-law after he barged into the office of a lawyer he believed was having an affair with his wife and brutally killed three people, the authorities say. He was sentenced to 30 years in prison for the crime.

But forgiven or not, a beaming Mr. Hanid Ortiz appears in a photo released by the State Department of 10 Americans and U.S. permanent residents newly freed from a Venezuelan prison as part of a prisoner swap.

At the time of the exchange, Secretary of State Marco Rubio said the prisoners had been “wrongfully detained” in Venezuela. But Mr. Hanid Ortiz has been less generous in his self-assessment, at least when it comes to his actions in Madrid, almost a decade ago.

“I am responsible for everything,” he wrote in the email to his wife’s sister, the authorities in Spain said when they were seeking his extradition from Venezuela, where Mr. Hanid Ortiz, a dual U.S.-Venezuelan citizen, had fled.

The killings took place in 2016.

Mr. Hanid Ortiz arrived at the lawyer’s office in search of a man he thought was having an affair with his wife, the Spanish authorities say. But he killed the wrong man, beating him to death along with a woman who was also in the office. He fatally stabbed another woman there.

“I did terrible things,” Mr. Hanid Ortiz, a U.S. Army veteran who was awarded a Purple Heart for injuries sustained in Iraq, said in the email. He said, “I lost my head.”

How Mr. Hanid Ortiz came to be on the plane with the other freed Americans, at least some of whom had been seized by the Venezuelan government as bargaining chips, is uncertain. President Trump is better known for his vows to expel ostensible criminals from the United States, not repatriate them.

The image of a convicted killer on a plane en route from Venezuela, surrounded by cheerful people waving American flags, is at best problematic for the White House.

“That runs counter to Trump’s message that he’s trying to purge this country of immigrants who are violent criminals,” said Jennifer Stromer-Galley, a professor at the Syracuse University’s School of Information Studies.

The State Department did not respond to questions about why the administration had decided to include Mr. Hanid Ortiz in the prisoner swap. It also did not say what happened to him after he arrived in the United States. But on Thursday, two people with knowledge of the case said Mr. Hanid Ortiz was now a free man.

Parin Behrooz and Julie Turkewitz contributed reporting.