The leader who ordered the attack is certainly evil. The question of the individual people that carried it out being is evil is more complex, something that can't be answered without knowing the people themselves. But in general, if the voluntarily carried out the attack then they probably fall into the side of evil.

The degree of evil would vary from person to person. It's very likely that some of the people where simple teens that got caught up in a religious frenzy and didn't really consider what they where doing. I would consider such people evil, but not nearly to the same degree as the life long terrorists that gets a kick out of hurting others or the leaders that worked out the attack to kill as many bystanders as possible.

Some of the people involved may have been duped or blackmailed into involvment, which renders their cases that much more complex.

If the line between good and evil was simple, even we humans would have figured it out.