Assholes exist the world over. Their presence may be one of the few universal things common to all human cultures. But I’ve come into contact each time I go abroad with a specific incarnation of douchebag that gets under my skin more than any of the others, perhaps because they usurp my own cultural identity and transform it into something heinous without my consent.
These people are The Ugly Americans.
They assume incorrectly in our own country that those who don’t speak American English perfectly and with no accent simply don’t want to learn, because when they go abroad they don’t bother to learn a new language. No matter that everyone else has to be bi or multilingual, if you don’t understand Amurrrican then you must be simply stupid. Or better yet, racist against anglophones (something of a semantic problem since we are not all of a single race).
They offer opinions for the whole of the US population when it’s really only theirs. They spout political rhetoric that makes no sense outside the bizzare land of the Republican primaries. They assume that all cultures share universal moral expectations, roundly condemning any who fall outside their own narrow definition of “good.”
They include all other people from the US in their asshorlery by sheer association, often consciously. They look to me and say, “We’re Americans, we…(i.e. think that China is the devil/think only the US has famous people/are Christians/don’t like Kimchi etc. etc ad nauseum),” expecting me to confirm through a wink and nod that all Americans think the same way.
They are purposefully, willfully, inexcusably ignorant and cling to that as a defining characteristic of “our” culture. You know who Montaigne was? Elitist. You form your own opinion instead of eating from the hands of news anchors? Radical. You have some shred of appreciation for other cultures? Amurrica-hater.
They are paradoxically fat and yet picky about their food. Growing up in a country where one has the luxury to refuse food simply because one doesn’t appreciate its taste turns Ugly Americans into whining overgrown children when they can’t find their precious favorite brand of jam. This combined with laziness–“You want me to walk across the street? What am I, a pro-athlete? There aren’t even escalators in this building! I might get out of breath!”–makes me wonder how the US would do if push came to shove and we no longer could afford to refuse calories and sit on our asses all day.
They are disingenuous and superficial.
They act as though theirs is the only way.
They jump from mild irritation to calling a person, company, or entire country racist in a single bound. One taxi driver passes them by because he’s already got a call, and he obviously was prejudiced. People stare on the subway, and they are clearly full of hatred for white skin and round eyes. The work website for a Korean company is in Korean? Racist bastards must be out to get us.
Despite their insistence on pointing out everything about every other culture that annoys them, even a kind word in jest pointing out their embarrassing and delusional behavior and they go on the warpath. They can dish out hours of criticism, but cannot fathom that someone else might have grounds to criticize their own behavior. Fastest route to having “UnAmerican” thrown at me, fastest route to hearing “racist” for those not raised in the US (and yes, apparently the American Race exists and somehow aligns with English-speaking).
Luckily, they only seem to be the majority of Americans abroad. In reality, the nicer people get drowned out in the drone of constant complaining and raging misplaced antipathy. I like to think that I am not part of their club, but grouped in with them against my will and found guilty by association. I wish that I could tell people to sit down, shut up, and start taking your lumps like the rest of the world.
But then, if I started openly arguing and complaining and carrying on…I would be associated with the Ugly Americans by more than proximity. Better to take my own advice, and quietly avoid them.
Reverse Retrograde 
We find ugly Americans all over the place, but don’t let it ruin your experience. I have met ugly Brits, ugly Germans, ugly Canadians, ugly Italians…the list runs ad infinitum. The one thing they all share is lack of experience and no way to compensate. Being flung into the middle of a foreign place is (awesome) but quite scary if you don’t know how do deal with it and Americans come with the distinct disadvantage of speaking only one language. Not only do we not understand the culture, we can’t even ask anyone to help us. Now, this does not hold true for you – multilingualism is a rather helpful trait.
Don’t be discouraged by Americans and cynical expats. As you said, it may be best to quietly avoid them.
Or just find the ones I do get along with! We all have our ugly sides.