I think greeting someone you've just been introduced to as 'Asshole' is undeniably unforgivable and tells your level of EQ (none)and IQ (none as well). There can be no acceptable reason for such behaviour, unless he really does have an anus for a face, (in which case you would do better by checking for a face where his/her asshole is), or he turns out to be someone you have no interest in maintaining any cordial relationship with (say, an insurance agent, or a tax collector).
What was the reason he gave?
That I had a face of pent up frustrations, and it was an attempt to help me let it all out via a burst of outrage. Fucking lousy attempt that is. No one has the rights or foresight to see an asshole of a person that he has met for the first time. Not even if he looks like one. And certainly no one has the rights to make such a one-off judgement on me if he doesn't know me enough to make a judgement. (If he is a friend that has known me for some time, there would be reason to justify his statement and I would be happy to introspect on why I'm an asshole in his/her eyes.)
I can only see a smirk smart alec at the insulting end, with foul excrements for brain contents. Lucky he stopped square dead when I challenged him, or this would be the 2nd time in my entire life that I've gotten into a fight. And that might indeed help me in venting some of my pent up frustrations/anger/rage.
But there are better ways to do this, and that is why many countries organise peace summits and peace talks with their enemies, instead of engaging in mindless bloodshed.