The word "justify" is quite ambiguous and possesses religious implications. I certainly do not believe that the argument that any terrorist attack is deserved is valid, but if you are writing an essay for an argument class, you do need to consider the other side. So, if you take the position that the 9/11 attacks were unjustified, try to include a concession in your argument to demonstrate that you are logical. Your concession could focus on the religious connotation of "justify." If Al Qaeda and its members believed that the United States' culture had perverted their religious culture and that the United States was responsible for the deaths of innocent Muslims, then they could argue that their response (9/11) was just and valid in their eyes of God.
Again, I certainly don't believe that committing mass murder is ever justified, but if you are trying to present a convincing, logical argument, you must do as your assignment details and discuss both perspectives.