The terrorists claimed they had a bomb. If all you have is a knife, people will rush you. If you claim to have a bomb as well, people will hang back, even if you kill one or two of them, especially if they don't know your plans.
There's some speculation that the plane that crashed in PA went down when the passengers found out on their cell phones what was about to happen to their plane. As long as they figured they might get out of that alive, they didn't want to upset the guys with the bomb. As soon as they figured out what was going to happen, they took out the terrorists.
It's what I would have done.