Donald Harvey, a former health care worker, called the 'Angel of Death' by the media after he confessed to killing several patients in Kentucky and Ohio in '70s and '80s, succumbed to his injuries Thursday, following an attack in his prison cell in Toledo, Ohio.

Harvey, 64, was serving 15 life sentences after he killed 37 people in 1987 while working as a nurse's aide at hospitals in London, Kentucky, and Cincinnati, Ohio. Later, he also claimed that he had killed 18 more people during his stint at the Veterans Administration Medical Center in Cincinnati. He was beaten up by an unidentified man in his prison cell Tuesday, a patrol report said. Officials found him lying injured in his cell, according to the Associated Press.

If Harvey would have survived, he would have been eligible for his parole in his '90s. Harvey used arsenic, rat poison and cyanide to kill the patients at the hospitals. Some of his victims died due to suffocation when their oxygen tanks ran out, New York Daily News reported.

He confessed that he performed his first murder while he was working at Marymount Hospital in Kentucky in 1970. After numerous murders, he was finally caught by a medical examiner who got the smell of cyanide while performing an autopsy. Harvey claimed earlier that he killed his patients to release them from their suffering. "I felt what I was doing was right. I was putting people out of their misery. I hope if I’m ever sick and full of tubes or on a respirator, someone will come and end it," Harvey told reporters in 1987.

However, former Ohio prosecutor Arthur Ney Jr. told the court in 1987, "He killed because he liked to kill," according to the New York Times.

After Harvey was arrested in 1987, his mother, Goldie McKinley, said: "He’s still a good boy. He’s just terribly sick and he needs a good doctor."

According to the Times, Harvey's co-workers gave him different nicknames and he also joked about killing some of his patients while he was on duty.

"He’d say, ‘I got another one today,’" the prosecutor told the court. "And everybody thought it was just a joke."

Harvey admitted to CBS News in 2003 that he had been able to kill so many of his victims so easily because "most of the doctors would be so overworked."

"So busy, that a patient could die and the family doctor would not come in and pronounce the person dead," he added. "They’d have a resident do that. They just pronounce him dead and send him straight to his funeral home."

One of Harvey's relative, Larry Bellamy, of Louisville, Kentucky, whose father-in-law was one of his victims told the Associated Press that he got what he deserved. "I'm kind of sad to hear it. But he was involved in about 50 people's deaths, and I guess maybe the good Lord gave him what he deserved," Bellamy said.