Aurors were trained to bring down their suspects, not intentionally wound or kill them. More powerful curses were only used in dire cases of emergency and Dark Magic was completely out of the question. Since in his mind, Moody was still in the role of an Auror, he followed Department regulations.
Moody gave it all he got, but between his advanced age and his disability, he was slower than his younger opponent... and Moody could tell. It made his temper simmer, but it was the Death Eater's words that made his blood boil.
A particularly fierce volley of curses came Moody's way, and the Auror couldn't block them all in quick succession. The last one got through his defenses and hit him in the upper part of his left arm. Moody didn't shout, he only grunted in pain and gritted his teeth. His eyes suddenly flared with a rage that spilled out from his mouth in the form of a curse that cut through the night, crackling with energy. Adrenaline and hate were useful tools, when directed in the right way, and Moody's experience was the filter through which he attacked the Death Eater.