Many viewers and readers were confused by the time when Mad Eye Moody turned Draco into a ferret when it was revealed that at the time that happened, Mad Eye Moody was actually Barty Crouch Jr. using Polyjuice Potion. The reason he did this was because he didn't like people who attacked from behind. He also likely did it as a form of retribution for Draco Malfoy's father who had lied about his allegiance to Voldemort's and never served time in Azkaban like Crouch Jr. did.