[@Alfhedil] [url=http://starwars.wikia.com/wiki/HK-series_assassin_droid#Known_models]Actually, according to the wiki the HK design was lost for some time until the Confederacy discovered it on Mustafar. They were going to mass produce them then Vader wiped out the engineers working on the project.[/url]

You could have your droid be one of the prototypes that got off the world and turned itself to a full, self aware bounty hunting droid? Or it could be one of the HK-77 units that was recovered from the world later and found its way into the criminal underworld?