They could incorrectly believe it's the truth. 9/11 does not prove that Wahabbist Islam is the one true faith.
The difference between the apostles and the 9/11 terrorists is that the terrorists died in faithful belief of something that someone else told them, while the apostles died for something they personally knew to be true or false. The apostles sincerely believed in the physical appearance to them of the risen Jesus. This is what they preached from the beginning and they went to their deaths proclaiming the Resurrection.
The fact of Jesus' death by crucifixion and the sincere belief of the apostles in Jesus' post-death appearances may be added to the following facts:
*The converstion of the church persecutor, Paul;
*The conversion of the skeptic, James (Jesus' brother);
*The empty tomb
to create a compelling argument for the historical fact of Jesus' resurrection. The opposing theories of skeptics cannot adequately account for the known historical data. The resurrection of Jesus is the most plausible explanation.