The last word was muffled somewhat by the sound of Rabastan's spell being deflected by the shielding spell that Moody managed to throw up just in time - years on the Auror force had given him quick reflexes, despite his age and physical limitations.
It didn't matter if Rabastan had cast a simple hex or the Killing Curse - it was all the same to him. He was being attacked! Moody's fighting instinct kicked in and he fired a stunning spell Rabastan's way, followed by a disarming spell to try to knock the wand out of his opponent's hand.