I fume when I hear 9/11 being labeled a 'tragedy', often by well-intentioned people who should know better.
When people die in an earthquake, tornado, tsunami or hurricane, it's a tragedy.
9/11 was a barbaric, unspeakable, premeditated act of mass murder, conceived and carried out by the most depraved of all scum, unworthy of the word human. It differed from Auschwitz only in its method and victims.
To label it a tragedy is to dishonor and trivialize those victims.