The Irishman, never shy of giving his harsh views on Sky Sports, admitted to deliberately harming ex-Manchester City ace Alf-Inge Haaland during the Manchester derby 20 years ago.
Norwegian ace Haaland, then aged 28, would never play another full 90 minutes again, after being on the end of Keane's horrific tackle.
The Manchester United skipper was initially handed a three-match suspension and a measly £5,000 fine.
But after Keane admitted to the horror challenge being deliberate in his 2002 autobiography, he was slapped with a further five-game ban and £150,000 fine.
For Bernstein, then chairman of Man City, neither punishment was enough.
He told The Athletic ahead of this weekend's Manchester derby: "I've never forgotten it. From a personal point of view, that was the worst individual thing I've been directly involved in, and the worst I've ever seen on the pitch.
"As a human being, it was an awful thing to see. Roy Keane stood over him and basically said, 'Take that, you b******.'
"It was done in cold blood. I have never forgiven Keane for that. I think, frankly, it's dreadful he's accepted in football the way he is. After doing something like that, I think it's absolutely appalling."
The 77-year-old added: "Whenever Keane turns up on television, I switch off. I just won't watch it. I'm appalled that he's still involved with football. It's just not right.
"Things happen, injuries do happen, but to do it deliberately and admit it the way he did, to sell his book, I think is completely beyond the pale."