His abandonment, cute looks and close relationship with the charismatic zookeeper Thomas Doerflein, turned him into a huge star. He became an environmental symbol, acting as a mascot for the German government's campaign against climate change and being superimposed into a photograph with Leonardo DiCaprio for Vanity Fair's Green Issue in May 2007.
But news of his premature death at the weekend has spurred on those who question both the way Knut was treated and the very fact polar bears are in zoos at all.
While polar bears can live to 30 years old, Knut was only four years and three months when he died. A post-morterm examination suggested the cause of death was brain damage, but already there have been accusations from animal rights groups.
- 2006: Born at Berlin zoo to 20-year-old mother Tosca
- Rejected by mother and hand-reared by keeper Thomas Doerflein
- March 2007: Debate about whether Knut should have been killed intensifies public affection
- 2008: Doerflein dies of heart attack, aged 44
- 2011: Knut dies
"Frankly, it would have been better for Knut not to have existed at all than live such a miserable life."
Those who questioned the implications of Knut's hand-rearing have suggested he suffered inevitable behavioural problems as a result both of his treatment and the crowds at the zoo.
But Linzey, author of Why Animal Suffering Matters, believes the issue is not whether the zoo was right to hand rear Knut. Once the cub was born, the management had a duty to hand rear him because a zoo is an artificial, "controlled environment".
The fundamental problem is wild animals being kept in captivity at all, he argues. "Zoos impose unnatural lives on most of their captives. People just see a cuddly bear and they want to gawk at him, but what they should see is an animal deprived of its natural life, exhibited for entertainment and profit."
Polar bears in the wild
- Born deaf and blind beneath the snow
- Adults spend most of their lives alone
- Polar bears use keen sense of smell to track blubber-rich prey, and are strong swimmers
"It does seem to highlight the dichotomy of people who love this one polar bear in particular and those who care about polar bears right across the species."
He sees little point in keeping large powerful animals in captivity. Not only do they lead "unfulfilled lives", but bears bred in zoos cannot be reintroduced to the wild as they lose the skills necessary to survive.
And those creatures bred in zoos become less and less like the wild animals we admire from natural history programmes, majestically leaping from ice floe to ice floe.
"As you breed in zoos down the generations you're getting further and further away from polar bear behaviour in the wild," argues Redmond. "You might be breeding out the traits that allow it to survive in the wild. What's the point? If you want cute cuddly bears for merchandising then that's a commodity."
In Knut's case critics suggested he had developed odd behavioural traits and had come to find the presence of the crowds necessary.
In recent years all but one British zoo has stopped keeping polar bears, a decision Redmond urges Berlin to follow.
Douglas Richardson, the zoo's animal collection manager, says they have learnt lessons from the past. In the 1980s polar bears became "the poster child" for anti-zoo movement after being kept in cramped concrete pits whose only attempt at recreating the bear's eco-system was white paint.
"I came up with a design that gives the animals between five and six acres of fenced off rolling landscape in the middle of the Highlands."
Richardson said that that much of the criticism of Berlin Zoo - such as over their merchandising policy - was unjustified.
"The European Zoo community pumps the money it earns from merchandise back into conservation in the field. I guarantee that when we have cubs the giftshop here will be full of fluffy polar bear toys and that money will be going to conservation. You have to take advantage of the situation. The money is not going to line someone's pockets."
The wildlife broadcaster Chris Packham acknowledges that a polar bear in captivity loses the ability to relate to bears in the wild. And he believes that if wild polar bears die out there is no point keeping some alive in zoos.
But he argues that zoos have a crucial advocacy role for animals in the wild. And if a zoo is treating the bear well - as he believes Berlin Zoo was with Knut - then keeping some in captivity is a price worth paying.
"We don't need many polar bears in captivity. But sacrificing those animals is justified as they become ambassadors for their species, striking awe into the hearts of humans. We don't want bears and tigers to go extinct."
The immediacy of zoo animals will always wow children and adults in a way that television documentaries cannot, he says.
"I can still remember aged 12 going to the zoo and seeing a tiger for the first time. I could barely speak I was so in awe of the animal."
That has benefits not just for raising awareness about wild polar bears but for dramatising the issue of climate change. Many people might find it hard to visualise abstract notions such as a two degree temperature rise in 50 years' time. But the polar bear losing the ice it relies on for hunting seals, is a story that we can all understand, he says.
"The long-term prognosis is tough for polar bears. So I'd argue that if Knut attracted a million people to see him and they were impressed by what they saw that is the most important role that a zoo can play."