I agree with what everyone has said here. Mustafa was at an age when he needed to sort out where he fit into the world of men, and all he had to go by was his memory of his father's rage, and then his father's death. There was a void. He wanted to have faith, he felt the lack of some clear and direct experience of God that would clarify his sense of purpose. and that made him vulnerable to a charismatic imam, who played on Mustafa's inherent desire to please and his tender heart.
Mustafa also wanted to please his friend Atef, his closest male influence and guide -- and Atef also pushed him to become more involved in this radical group. Mustafa knew it wasn't right for who he was at heart -- he struggled with his speech; he called himself a "roar without a bite." His heart wasn't in it, as strongly as he feels the injustice done to his people; but he felt he had to live up to what was expected of him as a man by the only men he was close to. He wanted to belong. And he felt a sense of obligation to others. And no one to show him a different way forward.