They destroyed your financial center and badly hurt your economy. The people inside were just collateral damage. They could have easily attacked stadiums or other areas with alot of people.
The US bombed al queda in the past and tried to assasinate bin laden. AL queda declared a holy war against the US and the US knew it. But you will probably say they are not "allowed" to declare war.
The Iraqi civilians stayed in Iraq because they did not have anywhere else to go. They hid in their houses as the US cluster bombed Baghdad. There are parts of cluster bombs stilll exploding in Iraq and tearing limbs off of children. 1 kid lost 13 members of his family, his legs and his arms. But you can just say sorry it was "collateral damage".
You think you are safer now because you murdered all of these people? AL Queda was not in Iraq before the war but they are now.
And when Bin Laden was blowing up the russians in Afghanistan he was the USs friend and a freedom fighter. But now he is attacking the US, because the US will not get their military out of the middle east, he is a terrorist.
The US calls it a "war on terrorism" so it is a war and any american civilians that get killed by them are just "coolateral damage".
And I do not call all militaries terrorists. If you are defending your country or attacking a country that attacked you it is different then starting a war yourself.