Because they’re foreign. And foreigners are either sinister or comic. Everyone knows that.
But this is the most important thing (because us Brits have standards to live up to):
The most wicked foreign accent of all was British English, according to the study. From Scar to Aladdin’s Jafar, the study found that British is the foreign accent most commonly used for villains….
Some shows also gave foreign accents to comic characters, though British English was almost never used in this way. “Speakers of British English are portrayed dichotomously as either the epitome of refinement and elegance or as the embodiment of effete evil,” the study concludes. “What general sociolinguistic theory would suggest,” Gidney added, “is that American adults tend to evaluate British dialect … as sounding smarter.”
Funny characters, on the other hand, often speak in German or Slavic accents (Dobrow offered as an example the associates of the evil Dr. Claw in Inspector Gadget), as well as in regional American dialects associated with the white working class. [yeah, whatever]
In The Atlantic.