Daniel's quest of revenge is his driving force, and the main reason that he allies himself with Rosh. He believes that he is entitled to his revenge because his father and uncle have been killed, and furthermore that he has a moral and ethical responsibility to fight the Romans for their occupation of Israel. In Chapter 7, he explains to Thacia:

"After my father died I made a vow. Maybe they would say a boy eight years old couldn't make a vow, a real one that was binding. But I did. I vowed I would pay them back with my whole life. That I would hate them and fight them and kill them. That's all I live for."
(Speare, The Bronze Bow, Google Books)

This has been Daniel's only passion for years. Daniel's mind has been fueled by revenge for so long that he cannot see any humanity in the Romans; they are a unified object to be fought and destroyed. It is only after he meets Thacia and Joel, and starts to see the disconnect between Rosh's banditry and actual useful action, that he starts to change his focus from blind vengeance to constructive rebellion.