By this point in the novel, Daniel is developing a conscience, largely due to the love and kindness of his friends, Daniel and Thacia, the dedication of his friend (the former slave) Simon the Zealot to the peaceful teachings of Jesus, and by the living example of Jesus himself. Jesus' kindness and humanity are beginning to be apparent to Daniel, and more appealing, when he compares Jesus to Rosh.
More literally, the reality of the task of harming another human being is repugnant. While Daniel had hurt others in the past, they had been attacked from afar. This deed is all too real, the pain he must inflict too palpable.
Finally, the old man whom Daniel is instructed to rob reminds the teenager of his own grandfather. Daniel's empathy for the eldery man is stirred; he moves his victim to the shade of a rock, and leaves the man one of the two weapons he carried in order that he may defend himself.