[michaelneale]

That sounds reasonable to me, it does seem darker in the British usage.

Also - in US English, you could call someone an "ass" - its not really that bad an insult, like you said, more about their behaviour at a point in time. There is also the donkey meaning (ass) which makes it less insulting then it would otherwise be.

[13ren]

Yes - the donkey meaning. I think actual donkeys might also have been more common in the USA.