Combat Droid on C-3PO's body was an NPC, with its head welded onto C-3PO's decapitated body. Since the head was the controlling part, it had the same mentality of any other Combat Droid: loyal and obedient to the Trade Federation and Separatists, hostile to the Republic and the Jedi.
Silence of the Clones Edit
Obi-Wan, Padme Amidala, and Anakin were all captured by the Separatists and sentenced to be devoured by beasts in an arena. At the same time Padme and Anakin were caught, C-3PO had been decapitated with a combat droid's head welded onto her body and her head welded onto a combat droid's body. In the arena, Obi-Wan, Padme, and Anakin had managed to break free from their chains, and kill all the beasts but the acklay. Jedi had arrived at the arena to rescue them, and combat droids had been sent in by Nute Gunray to fight the Jedi.
By this time, Padme and Anakin were riding the chariot they were carried in on, Padme was riding the beast pulling the chariot, while Anakin was in the chariot. Among the combat droids in the arena were the combat droid with C-3PO's head and the Combat Droid on C-3PO's body. Obi-Wan used his laser sword to slice off one of the acklay's arms. A droid shot the beast Padme was riding and she and Anakin both toppled to the ground. Padme suggested they burrow under the sand. Anakin was about to call the idea silly, but stopped himself because he didn't want to hurt Padme's feelings. Padme explained her idea, that arenas usually had underground tunnels, and Anakin could try to find them with his laser sword. Anakin realized that was actually a good idea and started to apologize, but stopped himself because he didn't actually call it silly, and he and Padme were tongue-tied.
During the course of the battle, the combat droid head on C-3PO's body had been sliced off and destroyed by a Jedi with a laser sword, leaving C-3PO's body headless. C-3PO's original head was then welded back onto her body by R2-D2.