Daniel Radcliffe shared his thoughts on a theory going around about the Dursley family's behavior toward Harry Potter and why they were as cruel and mean to their relative in J.K. Rowling's book series.
According to the theory, being in close proximity to Harry, Voldemort's accidental Horcrux, made the Dursleys moodier and meaner than they would possibly normally be - similar to how Harry, Ron, and Hermione acted when in contact with the Slytherin locket Horcrux in
Deathly Hallows. While Rowling has yet to comment on the theory, the star of the movie series voiced that it the idea had some merit to it.
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Daniel Radcliffe: “Yeah, that’s definitely the first time I’ve heard that one. It’s definitely interesting because it sort of posits a world where these people only known to be a certain way. Actually if you remove him from the situation, [they] would have been quite a nice, friendly family.”
“Having been there for Richard Griffiths’ (Vernon) and Fiona Shaw’s (Petunia) performances. I feel like it was more they just hated having this kid around.”
“But I think that’s definitely a cool theory,” he added. “Has anyone asked Jo Rowling what she thinks of that one yet? I feel like that one ― that’s definitely a good one.”
It should be noted while this is a reasonable theory,
Rowling delved into the Dursleys' history many years ago, revealed on Pottermore, which more or less contradicted the Horcrux theory.
Rowling noted that Petunia's bitterness and anger stemmed from her childhood, never having become a witch, and she veered toward Vernon Dursley, who was as anti-wizard as possible. They were both like this well before Harry as an infant showed up at Privet Drive on Halloween, 1981:
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Petunia Evans, forever embittered by the fact that her parents seemed to value her witch sister more than they valued her, left Cokeworth forever to pursue a typing course in London. This led to an office job, where she met the extremely unmagical, opinionated and materialistic Vernon Dursley.
Vernon was apt to despise even people who wore brown shoes with black suits; what he would make of a young woman who spent most of her time wearing long robes and casting spells, Petunia could hardly bear to think.
Reading the shocking contents of Dumbledore's letter, however, which told her how bravely Lily had died, she felt she had no choice but to take Harry in, and raise him alongside her own cherished son, Dudley. She did it grudgingly, and spent the rest of Harry's childhood punishing him for her own choice. Uncle Vernon's dislike of Harry stems in part, like Severus Snape's, from Harry's close resemblance to the father they both so disliked.
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Daniel Radcliffe on theory that Dursleys' hatred came from Harry Potter's Horcrux 
