Rosh has lived in the hills for years, stealing from merchants and caravans, and recruiting from those who wish to fight the Romans. He claims to be on the side of Israeli independence, and that is what attracts Daniel to his band. Daniel, however, is a true patriot because of his desire for revenge; while he sees Rosh as a father figure and an inspiration, he realizes when Rosh refuses to help rescue Joel that the only thing Rosh cares about is himself.
In another moment Rosh would spring at him... He could not fight with Rosh. The very strength of his hands he owed to this man. And what good would it do Joel? He turned away without speaking, and passed through the ring of silent, wary men. He knew that he was through with the mountain for ever.
(Speare, The Bronze Bow, Google Books)
This is a turning point for Daniel, who discovers that his friendship with Joel is more important than his need for revenge. If he stayed with Rosh, he could eventually battle with the Romans, but at the cost of Joel's life. This realization leads him to leave Rosh and form his own group, one dedicated explicitly to freeing Israel. Without that turning point, Rosh's self-interest would probably have continued to influence Daniel until he no longer cared about his cause, but instead cared only for himself.