Put yourself in the shoes of the other passengers on this plane. The very guy who wanted thousands of Americans to die was ALLOWED on the plane with you and your children???? Then the door was closed and pulled away from the gate??
If he was indeed (as you say) allowed on the plane, then heads should roll for risking a hostage situation.
Why?
He was screened like any other passenger.
Could he have crashed a plane that was still taxi-ing (sp?) with the cockpit doors locked?
The possible gain may have been deemed to be worth the increased risk.