Mr Hillary, son of Sir Edmund, the first man to conquer Everest, came off K2 yesterday and told of the obsession that led to the deaths of seven climbers on the world's second-highest peak. In an exclusive interview with the Independent, Mr Hillary, 40, said that a bizarre chemistry had developed among the climbers who headed for the summit on 13 August.
Mr Hillary and the remainder of his expedition, Matt Comeskey, 31, and Kim Logan, 43, all from New Zealand, arrived here in Pakistan's Karakoram Range after trekking down from the mountain. They lost two of their team, Bruce Grant and Jeff Lakes, when appalling weather closed in on the mountain. But Mr Hillary, who turned back before disaster struck, said he believed the doomed climbers ignored signs of dangerous weather ahead.
He turned back at noon on the 13th because he saw two weather fronts building from the north and south. The others continued for six hours before reaching the summit and being swept to their deaths by winds.
"Summit fever had developed in that group," he said. "There was a chemistry in there that meant they were going for the summit no matter what ... They were all driving each other on. These people came together and because of the place and the atmosphere and their personalities, they became blinkered and simply focused on the top. There was no careful awareness in the group and the most dangerous thing about groups is that everyone hands over responsibility for themselves to someone else ... It means that no one is taking responsibility. There can be a false sense of strength in numbers, but it doesn't matter how big your group is - you can have 1,000 people and the mountain could still kill them all."
Ms Hargreaves, the American Rob Slater,the New Zealander Bruce Grant and three Spaniards, all from three combined expeditions, died after deciding to head for the summit as light was fading and poor weather was closing in. The seventh victim, the Canadian Jeff Lakes, did not reach the top and made a heroic but vain attempt to come down.
Mr Hillary had rested at Camp 4 on the morning of the 13th having met Ms Hargreaves on the way up. "It was desperately cold up there and I had come down to warm up," he said. "I saw Alison and she simply said, 'I'm going up.' Nothing was going to stop her. She was quite obsessive.That really is her story.She was going up and that was that."
After resting at Camp 4, at about 26,000ft, Mr Hillary began the final ascent. Ahead of him, he could see Ms Hargreaves and the rest of the group tackling the Bottle Neck, a huge and extremely dangerous funnel of ice.
"Alison shouted down, 'Come on up. Use the red rope,' " said Mr Hillary. "I was climbing with Jeff Lakes and he decided to go for it. But I just sat there and looked at these dots climbing ahead on the traverse. I was amazed because Alison was crawling down as cloud drifted across the face. I could only see her intermittently.
"They had all become blinkered by the summit. They had become obsessed by that but I had become obsessed by the huge cloud banks that were building up in the north.
"I turned back at midday and even then I was finding it difficult to find my way down over relatively easy ground. It was a mistake for them to go on the way they did."
Despite increasingly ominous weather, the other climbers continued and all except Mr Lakes reached the summit shortly after 6pm. That meant they would have to return in darkness, something most climbers would never contemplate doing.
"The weather was getting worse all the time but I made it to Camp 2 at 22,000ft at 7.30pm," said Mr Hillary. "When I was told that they had summited an hour earlier,my response was simply 'Oh my God'.
"When you are on a summit over 8,000 metres and it's dark, it's like being in space with no way back. You might as well hitch a ride from a passing spaceship. I knew they were in for a bad time.They would either die or bivouac up there, in which case they would have probably lost their legs to the cold." Of the seven, only Mr Lakes, 33, had abandoned the climb and tried to return. He made it to Camp 4 but was buried in an avalanche. He dug himself out and staggered down to Camp 3 only to find it destroyed by the wind and snow.
"We were in radio contact with him but we didn't think he would make it," said Mr Hillary. "The only way down was by a vertical 2,000ft rock bluff, but, incredibly, he made it."
Matt Comeskey dragged him into his Camp 2 tent some 30 hours later. He was exhausted and dehydrated but free from frostbite.
"He was having difficulty breathing but we warmed him up and steadied his breathing after three to fours hours," said Mr Comeskey. " We thought he was fine and then we went to sleep. The next day - a weird, sunny beautiful day - I woke up and he was dead beside me. It seems he died from sheer exhaustion."
Mr Hillary's team buried Mr Lakes's body on the mountain. The other six are likely never to be recovered. "What happened was a tragedy and was terrible to watch, but you could see it unfolding," Mr Hillary said.
"Alison was a brilliant climber but she had tremendous commercial pressures on her and she became obsessed.When you spoke to her it was clear that climbing came first and everything else was secondary.
"Rob Slater was determined but very inexperienced - he had never climbed an 8,000-metre peak before. Then there was one of the Spaniards who was young but very, very strong. They all drove each other on and the others followed.
"In a way it all started snowballing in on itself. It was so clear that this big, black nasty-looking bank of cloud to the north was coming in and prudence said withdraw. I did and I hoped they would do the same. But they didn't.
"They were a strange mix of climbers on that day and they are all to blame. They all had the opportunity to back off, but they chose not to."