7D6 Protocol/Assassin droid
The droid had secrets. It had the typical 7 million languages of all protocol droids, but it also had something else. It was designed as an infiltration/assassin droid, and this particular model had two heavy-explosives implanted in its legs. One of these explosives had enough power to take out a Star Destroyer, both had enough to destroy a SSD. His Imperial programming still existed with in his droid-brain. It was just a matter of time for it to be re-triggered and he in turn become loyal to the Empire once again.
The fact he went renegade and why is also a mystery? Apparently his maker dropped an AI virus in him, and also implanted several targets for the explosives to be triggered off of, one being Darth Vader himself. Unfortunately, his plans did not completely come to fruition.