Question: why does ruzek get arrested?
Answer: When Antonio pushes a criminal out of a window during a fight in a drug haze, Voight sends Antonio away and tells Ruzek to pin it on him. However, Ruzek, aware of Kelton and Voight's feud, takes the blame on himself. Kelton does not let Internal Affairs drop the case, and in the final episode, Ruzek is arrested.