I think that the justification depends on the extent of whatever actions you're planning to take against them, but yes, if that group presents an ongoing threat to my my country, I would say that moving to stop them from posing such a threat is fully justified.
So, if you live in say Afghanistan, have only limited information, people around you are starving, dying due to missing medicaments, you see another country supporting attacks against fellow muslims (Israel?), maybe you know someone who was killed there. The threat of a forceful invasion constantly hanging in the air, so that more killing will happen, either in your country or another country.
Wouldn't you like to root out the Problem? Attack the center of the enemy military (Pentagon), the people who cause your friends to die through economical means (WTC) and their leader (White House) to stop this world wide terrorism?
You know, i think that wouldnt be justified. And, I am quite sure you wont think that its justified. But is the difference between those people, who thought they were attacking evil murderers, and someone, who supports attacking some people who are maybe evil murderers really that great?
The difference between the good guy and the bad guy in a fight is that the good guy keeps to rules, even if it is to his disadvantage. How can we tell others about human rights, if we dont care about themselves? (Guantanamo) How can we tell others, to let us live in peace, if we are attacking their homes, their families? How can we tell others not to violate international law, if we dont care about it?