Try homegrown terrorists. just look at OK City if you are wondering what I am talking about. there are loads of private militias out there, both para-military and religious, and any of them can foster completely blind devotion like that given a charismatic leader.
If nothing else, read up on a few and make up your own for the film. Each group has a focus of some sort. Give it some thought and you can come up with all kinds of twists. If the movie is from the terrorists point of view you can build empathy with them by showing the perceived tyranny they are facing, as well as animosity toward those who are hunting/persecuting them. Just imagine that you are trying to put out propaganda for this imaginary group and go for building up their "noble cause" and the "cruel deceit" of the opposition.
I could go on, but I will leave it to you.