The HK-50 series of droids were based off of the earlier HK-47 assassin droid. Like their progenitor, these droids possessed incredible linguistic skills and highly sadistic tendencies. The primary function of the HK-50 droids, as they stated it, was to "facilitate communications and terminate hostilities." In accordance with the usual perceptions of HK model assassin droid, the best way to terminate hostilities is to kill everything present. HK-50 units are identical in appearance to HK-47, with the primary exception being that they are gray in color instead of HK-47's rusty orange.
The HK-50 droids were manufactured in secret by the droid G0-T0. They were programmed to hunt Jedi, specifically the Jedi Exile. HK-50 assassin droids were often able to successfully pose as protocol droids and they infiltrated much of the Republic fleet in doing so. Negotiations where HK-50s were used to translate often did not conclude well, as their cruel and violent desires influenced their interpretations of the various spoken languages. The HK-50 units hidden in the fleet were looking for the Jedi Exile, who was believed to be the last of the Jedi. When the Exile became a passenger aboard the Harbinger, an HK-50 droid on board the Republic capital ship took note. The HK-50 unit forged orders that the Exile report to the medical bay for automated and mandatory injections. The Exile was injected with a dose of sedatives that would be lethal to a non-Jedi, but which would only render him/her unconscious. The HK-50 locked up the Exile and arranged a series of accidents aboard the Harbinger intended to disable it and allow for retrieval of the prize. The plans of the HK-50 were foiled when Kreia, flying in the Ebon Hawk, was brought on board the Harbinger and took the Jedi Exile. The Ebon Hawk had been pursued by and fired upon by a Sith capital ship. When the Sith ship was investigated, cloaked Sith assassins boarded the Harbinger and seized it. Kreia and the Exile escaped the ship on the Ebon Hawk, but the HK-50 unit tagged along.
The Ebon Hawk, severely damaged, made its way to the Peragus Mining Facility. Once there, the HK-50 again posed as a protocol droid and arranged for a series of accidents to befall the facility. The Jedi Exile was again drugged into unconsciousness. Division among the miners about what to do with the Jedi (the Exchange had a large bounty out for Jedi) was manipulated by HK-50 to kill all of the facility's workers. HK-50 also corrupted the programming of the facility's mining droids, allowing them to "mine" organics. The subtle manipulations of the HK-50 unit and the highly fatal results demonstrate the dangerous nature of the HK-50 model of assassin droids. Before escaping from the facility, the Jedi Exile and company destroyed that HK-50 unit.
After Peragus, the Jedi Exile continued to run into HK-50 droids throughout his/her travels. After the previously damaged and inoperable HK-47 was reactivated, it was able to discover that the HK-50 droids were tracking the Exile through a sonic imprint sensor based off of the vocabulator of the HK-50 from Peragus. HK-47, wanting to track down the HK-50 droids and their source, suggested that the tracking device be left intact until the group was again confronted by HK-50 units. Because the HK-50 units were made from HK-47's template, he could not harm them himself due to his self-preservation protocols (on the other hand, neither could they harm him). HK-47 therefore had to rely on the Exile to fight the HK-50 droids and possibly capture one. When HK-47 finally found the factory, the HK-50 units allied themselves with him in order to destroy G0-T0.
Behind the scenes
Though the HK-50 droid factory on Telos was cut from the game due to deadline constraints, HK-47 was supposed to discover its location. HK-47 would have gone to the factory and, having been unable to persuade the HK-50's to join the cause of the Exile, would have enlisted the help of the (not quite bug free) HK-51 droids. The HK-51 units would later come to HK-47's aid against their manufacturer, G0-T0, in a final confrontation on Malachor V.