He believed in a world government and he was extremely sympathetic to Hitler, Mussolini, and the entire Nazi cause. His stories were filled with caricatures of greedy Jews.
I thought he had been a pilot in World War Two.
Cory Doctorow also weighs in:
I've always heard that children's author Roald Dahl was something of a creep in person, and knew that he had a reputation for doing library visits and conference appearances that turned into antisemetic rants. But this biographical sketch of him at This Recording depicts him as even creepier than suspected -- a cruel, womanizing bully, ruled by lust and petty jealousies. I continue to enjoy his fiction with my daughter (even as I worry about their misogynist subtexts), but it's a shame to learn that an author who's given me so much pleasure and wonder was also such a rotten guy.
Like Doctorow, I still plan to read Dahl's stories to my son, but perhaps I will be examining them a little more closely.
Of course, I have heard the same of other authors. I enjoyed Enid Blyton's stories as a child, but she wasn't a pleasant person. From The Telegraph describing a BBC biography of Blyton:
One of the most telling scenes in the film features a tea party that Blyton has organised for a group of her young fans. While the writer makes a fuss of the visitors, her own children are watching from the house where they are locked away from view.
Dickens and Naipaul, too, do not shine in close up:
If Dickens sometimes behaved badly, Naipaul is unquestionably a bad man, notorious for his floridly abusive relationships and bigoted ideas. Does this diminish his work? Naipaul's fiction is not to everyone's taste, but the grace of his prose and the power of his early books, especially "A Bend in the River," is hard to deny; I admired much of that novel even as I gritted my teeth over its blinkered depiction of Africans.
I have heard unpleasant things about Farley Mowat, a hero of mine while growing up. His evils seems more of excess, of being, or trying to be, bigger than life.
It seems people often become famous for one thing and then their views on other fields are sought, then trouble begins. It reminds me of the warning, "Don't talk politics with your surgeon."