I think the two concepts just aren’t that far apart in American English, and words can drift back and forth. Think about insulting the driver in the next car — it’s not always clear if you’re talking about their intelligence or lack of empathy and someone listening might get the opposite impression.

Clown can mean jerk in either sense. “Who’s this idiot?” means something closer to asshole. In old movies you’ll hear someone called “a selfish fool” or “inconsiderate fool,” where fool means something closer to asshole than fool as we usually use it.

Maybe the underlying issue is that it’s hard to tell someone acting obnoxiously because they don’t know better from someone deliberately indifferent or malicious, and the consequences are often the same.