Walter Bonatti was an Italian mountaineer who became a vilified figure in one of the most enduring controversies in his sport — his role in the first successful ascent of K2, the world’s second-tallest peak. Mr. Bonatti, who died Sept. 13 in Rome at 81, was vindicated more than 50 years after the 1954 summit of K2.

Britain’s Guardian newspaper, which reported the death, said he had recently learned he had cancer.

“Bonatti was an astonishing climber, probably the greatest of all time, in my view,” Jon Krakauer, the author of the best-selling book “Into Thin Air,” wrote in an e-mail to The Washington Post. “He led a remarkable and [in certain regards] tragic life.”

Raised in a village in northern Italy, Mr. Bonatti began climbing as a boy with his friends, using their mothers’ clotheslines for rope. His prodigiousskill allowed him, at 19, to ascend the Walker buttress of the French-Italian Grandes Jorasses.

Mr. Bonatti’s confidence and athletic ability made him an obvious selection forthe 1954 team of Italian climbers bound for the 28,250-foot K2 in the Himalayas on the Pakistani border.

Being the youngest of the group, Mr. Bonatti was passed over when the time came to make an attempt for the peak. Instead, the task went to veteran climbers Lino Lacedelli and Achille Compagnoni.

Mr. Bonatti and his Pakistani porter, Amir Mahdi, were charged with carrying oxygen tanks for the two older Italians to use in their final push for the summit. But Mr. Bonatti never met up with Lacedelli and Compagnoni.

Mr. Bonatti claimed that the two had abandoned him and Mahdi on the mountain.

While Lacedelli and Compagnoni slept in a protected tent, Mr. Bonatti and Mahdi were forced to sleep in the open at 26,600 feet in subzero temperatures. Mahdi lost fingers and toes to frostbite. Mr. Bonatti, unscathed, left the tanks in the snow and descended in the morning.

That day, July 31, 1954, Lacedelli and Compagnoni made history as the first men to reach the top of K2.

When Mr. Bonatti returned from the Himalayas, he alleged that Compagnoni and Lacedelli had tried to kill him by giving a false rendezvous point.

Compagnoni told Italian newspapers that Mr. Bonatti was an ambitious youngster who had attempted to sabotage the expedition’s success by siphoning off the air in the cylinders.

Compagnoni’s reputation led many Italians to believe his side of the story, and Mr. Bonatti became an outcast to many members of the climbing community. He claimed that fellow Alpinists punctured the tires on his car and set fire to his home.

“They never really accepted me,” Mr. Bonatti told the Guardian last year. “Luckily for me, they also didn’t realize I had already sold the house.”

The question of why Mr. Bonatti never met up with Lacedelli and Compagnoni at the end of the K2 climb lingered as a mystery for decades.

In 2004, Lacedelli published a memoir that substantiated Mr. Bonatti’s version of events. Four years later, the Club Alpino Italiano, the governing body of Italian climbing, recognized Mr. Bonatti’s crucial role in the expedition. Lacedelli and Compagnoni died in 2009.

“Bonatti was one of the all-time giants,” Alison Osius, executive editor of Rock and Ice magazine, said in an interview. “But you certainly can’t separate him from the K2 ascent. He should have been there, and he really was robbed.”

Walter Bonatti was born June 22, 1930, in Bergamo, a town in northern Italy. Survivors include his wife, Rossana Podesta, an actress who played the title role in the 1956 action drama “Helen of Troy.”

After the K2 episode, Mr. Bonatti charged ahead as one of the finest alpinists of his generation. A purist, he refused to change his style when new technology became available. He often climbed using hemp ropes and wooden pitons.

“Every climb I did was about challenging myself, about not knowing if I had what it took to survive,” Mr. Bonatti told the Guardian. “I seldom felt a feeling of great triumph when I made it to the top; that feeling came when I was on the mountain itself and I know there was nothing that could stop me.”

He went on to conquer many of the world’s most imposing peaks, including a first ascent on the Petit Dru in the French Alps. He spent six days and nights on the rock face while climbing what later was named the “Bonatti pillar.”

That Mr. Bonatti completed many of his most harrowing ascents alone was a decision that he traced to the betrayal he experienced on K2 and that he chronicled in his 1998 memoir, “The Mountains of My Life.”

“My disappointments came from people,” he wrote, “not mountains.”

According to Krakauer, who wrote the introduction to Mr. Bonatti’s memoir when it was published in the United States in 2001, the Italian climber was the hero of the K2 expedition.

“Without his superhuman effort in support of the two guys who reached the top, the summit wouldn’t have been achieved. No way,” Krakauer wrote to The Post. “And then Compagnoni repaid Bonatti’s near-fatal sacrifice by totally slagging him in the Italian media. It’s one of the most shameful incidents in the history of mountaineering.”