Daniel initially joined with Rosh because he thought the bandit leader could offer him something that he desired: revenge. Daniel assumed that Rosh was working for the cause of freeing Israel from Roman occupancy, because his criminal activities often worked against Roman interest. In fact, Rosh has always been a criminal, working entirely for himself, and he used the cause of freedom as a way to further his own interests. Daniel finally realizes this when Rosh refuses to help him rescue Joel.

The red mist of anger cleared suddenly from Daniel's mind. He looked at the man who had been his leader. He saw the coarsened face with its tangle of dirty beard. He saw the hard mouth, the calculating little eyes. He saw a man he had never really looked at before."The cause!" he said with despair. "How could you know what it means?"
(Speare, The Bronze Bow, Google Books)

Joel is Daniel's friend, one who has helped him rediscover his own humanity and his capacity for love. To Rosh, however, Joel is simply a pawn to be used and discarded when he is no longer of use. Sacrificing men to save him on moral grounds -- which is exactly what Daniel does later -- is alien to Rosh, since he cares about nothing except for his own survival and comfort. Daniel finally realizes that Rosh has been using him since day one, with no actual concern for the cause itself.