Assassin droids were droids constructed initially for law-enforcement for dealing with particularly dangerous criminals and later were developed for the purpose of killing sentient beings on behalf of crime lords, corrupt politicians, and bounty hunters.
Originally, assassin droids were developed in the Old Republic as a law-enforcement droid designed to keep the peace by either capturing or otherwise killing violent criminals. Several were later repurposed as war droids for the Outer Rim frontiers as well as the Corporate Sector.
When a crime lord, corrupt politician, or bounty hunter required an individual to be eliminated, they sometimes deployed or hired an assassin droid to execute their will. These droids generally performed with unflinching loyalty to their masters, but rogue assassin droids were not unknown. Since their missions generally required a far higher degree of independent skill, savvy, and judgment than normally possessed by simple war droids, their programming subroutines, circuitry, and memory cores were of correspondingly higher sophistication and quality.
Equipped with an array of weapons, assassin droids were also used as elite infantry when the need arose for skillful and quickly adaptable soldiers with unquestionable loyalty, as with the Confederacy assassin droids deployed during the Battle of Jabiim.
The planet Caprioril was the site of the infamous assassin droid tragedy shortly before the fall of the Old Republic, in which a droid slaughtered 20,000 humanoids and aliens at a swoop arena in a massive explosion in order to assassinate Governor Amel Bakli. Because of this, as well as the assassination droids' gradual misuse by politicians, crime lords, and warlords, there were attempts by the Galactic Senate during the waning days of the Old Republic to have them banned, although this ultimately failed. Ironically, it was during the rise of the Galactic Empire that Emperor Palpatine ordered all assassin droids banned throughout the galaxy due to their constant use against himself and his forces, with it being largely more successful. However, despite the Imperial ban, another Imperial, that of Holowan Laboratories Supervisor Gurdun, ordered the creation of the IG-88 assassin droids, which later betrayed both him and his forces.
In 14 ABY, assassin droids flooded the market thanks to Lannik Racto, a crime lord on Coruscant, but his operation was ended by Jedi Jaden Korr. These assassin droid models featured hand energy weapons and a powerful nimbus of electricity that delivered a near-fatal shock upon contact. The droids could not fire and maintain the nimbus at the same time, however.
An inordinate number of assassin droids achieved sentient independence, including IG-88 and C-3PX. This is presumably due to the higher level of autonomy that is incorporated into an assassin droid's droid-"brain" processor design since independence is a functional requirement. These droids with no masters typically fund their independent existences through the use of their innate abilities, plying their trade as bounty hunters.
One year after the First Battle of Geonosis, General Obi-Wan Kenobi was sent to Cestus to investigate the production of the JK-13 droid line by Cestus Cybernetics. The JK-13 was also known as Jedi Killers among the criminal classes and possessed shields. The JK droids were sold for almost a year throughout the galaxy.
Listed in Peterson's Guide to Droids of the Republic, Vol. VII: The Great Corporate Expansion Era, the Tac-Spec Corporation FIII Footman droids are considered legendary, among the deadliest personal service units ever created combining fanatical loyalty and an impressive kill ratio. These droids were both enormously expensive and on a limited production run.