Most brazillians did not celebrated 9/11, obviously. Most did not care.
Several of those who did, were shocked, and sad, specially those that had relatives there.
But several were VERY HAPPY, usually those that got screwed during the cold war.
My point is: people forget, that when you make yourself a hated enemy, people WILL hate you, in irrational ways, it is easy to create terrorist and a vicious cycle of violence.
Brazil up to WWII liked the US, bought craploads of US stuff, and even joined WWII for the US side despite the president being fascist and having connections to the Italian fascists (of course, the fact US offered a steel mill in bribe when Brazil had none, was a huge factor).
During cold war, US backed the military coup here, and quickly things turned into "we vs them", we had during the cold war a US embassador kidnapped, citibank branches bombed or torched, english speaking soldiers killed (the most famous was two UK navy sailors that were here on vacation) and so on.
What make you think, that after you make people believe you are their enemy, that you are at war with them, they will stop believing that after situation change?
There are lots of people here that has a grudge not only with US, but with all of its citizens as well, injust as that might be, the destruction of the World Trade Center, a economic symbol of the US, and the attack on Pentagon, is something that several people around the world cheered (obviously more on middle east, where people threw wild parties in the street and shot upwards) because those people viewed it as the invincible bully being hurt for the first time, people saw it, horrible as the actual event was in numbers of civilians killed, as a successful war attack, revenge, justice, david hurting goliath, etc...
People are NOT psychopaths, they are normal people, hurt by US government so deeply, that any chance they have to see it hurting back will make them happy, even if it means the deaths of thousands of innocents.
This is what fuel wars...
United States huge military is a deterrent for countries attacking it, but don't mistake that peace for the world agreeing with the US, I am very sure few countries would side with the US in a hypotetical world war with a level amount of military (ie: not US having alone half of the world military power, as current it is).
I am very sure that if you leave a US citizen (any citizen, the most adorable citizen even) near one of the territories were PCC is popular (PCC is a huge organized crime thing here, created during the cold war with purpose of fighting US-backed government), people (normal people, civilians, not members of PCC) will be happy to do things to that citizen that is best not describe here...