Whilst I agree with your point, I am shocked at your courage to attempt to use the English spelling of "arsehole" in the internet. Haven't seen it written that way for too long a time !
I find myself giving up and using American spelling and terms.
Even Firefox doesn't know how to spell, despite me telling my OS many times my locale to an alarming level of detail.
I think you're right, that it's simply US vs. British spelling, but I've always felt that the usage is slightly different (and that therefore maybe they are actually different words...).
An asshole seems to be someone you are angry with (legitimately or not). It's about behaviour (or perception thereof); but to call someone an arsehole is a darker, earthier insult, and is closer to what they intrinsically are. It seems more literal to me.
Maybe this is because I've mostly heard "asshole" on sanitized TV; but I've heard "arsehole" in uncensored RL.
PS: Firefox then spellchecked in a way that was almost but not quite entirely unlike British English.
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.
I find myself giving up and using American spelling and terms.
Even Firefox doesn't know how to spell, despite me telling my OS many times my locale to an alarming level of detail.