Vengefulness is a real emotion, as the grandparent implies. It leads you to take pleasure in the pain of people who you feel have harmed you or something you care about (see: most of the justice system). There are also people who take pleasure in anyone else's pain. They are too few to matter much in explaining an organization the size of Al Qaeda, never mind "the terrorists" or "religious fanatics".

Now, there may be confusion here about the phrase 'because of our freedom'. There are moral reasons, and then there are causes. I don't think Al Qaeda members perceive themselves as taking revenge on the West for the weakness of the Muslim world, nor as serving the house of Saud. I also don't believe either Al Qaeda or Isil would exist if the Muslim world were stronger relative to the West, nor that they'd exist without the machinations of the house of Saud. And I usually couldn't care less about terrorists' "moral" reasons.