(CNN) Prison officials faced significant criticism last year when infamous mob boss James "Whitey" Bulger was transferred to a new prison and then almost immediately killed by other prisoners.

But in a new interview with NBC News, Bulger's former prison warden deflected blame and put the onus on the notorious Boston gangster himself.

"Quite frankly, I think he wanted to die," Charles Lockett, the Florida penitentiary's former warden, told NBC News . "I think whatever issues he had, he had come to peace with them."

"It's a tragedy, but I don't think anyone was deficient in their duty," Lockett, who retired in late December, added.

The comments come six months after Bulger, 89, was transferred from a federal prison in Coleman, Florida, to the Hazelton prison in West Virginia and placed in the general inmate population. He was killed by blunt force injuries to the head a day later, according to his death certificate.

Former prison officials and experts have sharply questioned the decision to move a well-known figure like Bulger into a prison with lower security, particularly given his health issues.

Joe Rojas, president of American Federation of Government Employees (AFGE) Local 506 at the Florida prison, said prison officials "dropped the ball" by transferring him to the notoriously violent Hazelton facility and mixing him with the general population.

"It's like sending somebody to death row," Rojas, who was not involved in the decision to transfer Bulger, told CNN in November.

Cameron Lindsay, a former warden at five Bureau of Prisons facilities who said he had no direct knowledge of the case, also questioned the timing of Bulger's transfer.

"The BOP intentionally designated and transferred Bulger with the belief in mind that he could walk the line at Hazelton, meaning he could be placed in the general population there," he told CNN. "This is astounding given the fact that Bulger is a super-high-publicity case, as he's one of America's most infamous criminals."

Lindsay said, "I don't think that any nefarious plot was afoot. That would be terribly inconsistent with what I know about the Bureau of Prisons. I think it's more -- I'm going to guess it's an issue of complacency. Somebody was asleep at the switch."

Why Bulger was transferred

Bulger, leader of the South Boston gang, was the ruthless kingpin of a criminal empire accused of a number of brutal murders. His infamous reign would be the inspiration for Jack Nicholson's character in Martin Scorsese's film "The Departed."

Bulger evaded police for 16 years until his capture in 2011. A federal jury convicted Bulger of 31 counts, including racketeering and extortion, and found him culpable in 11 killings. He was set to spend the rest of his life in prison.

Bureau of Prison records obtained by CNN last year state that Bulger suffered from high blood pressure, aortic stenosis and prostate and bladder problems.

A previous request made by officials at Coleman to transfer Bulger was denied in April. Records show the request was coded to indicate Bulger required a higher level of medical care, or Care Level 4, for inmates who are severely impaired and may require daily nursing care.

It's not clear why the April transfer request was denied. In October, however, a transfer request was approved and coded to indicate he no longer needed to be in a medical facility.

Records obtained by CNN show the former South Boston mobster was less than a stellar prisoner. In March 2018, Bulger was found guilty of disruptive conduct, according to the records. Rojas said Bulger was sentenced to 30 days in a special housing unit after threatening a nurse in February with the warning, "Your day of reckoning is coming."

Lockett, his former prison warden, told NBC News that the threat came after medical personnel recommended Bulger be taken to a local hospital and be seen by a heart specialist.

"She pressed him to go see the doctor, and he got mad about it," Lockett said. "He told her point blank, 'I know people. I still have connections back home.'"

The Federal Bureau of Prisons said last November it doesn't disclose the reasons for specific inmate transfers. "Per BOP policy, the designation process includes a review (that includes) whether there exists any known safety threat from other inmates," the prisons bureau statement said.

"There were no known threats to Mr. Bulger from other inmates upon designation to the Hazelton facility."