The Star Wars franchise is populated by a variety of aliens and droids alike, and there seem to be equal varieties of the other. The whole plot of the original movie in 1978 was based on the escape of a droid, and that same droid acted as a magical McGuffin several times throughout the plot of all three movies to open hatches, unlock doors, and fix hyperdrives on the fly.
Droids can take on virtually all of the same professions, hobbies, or lifestyles as their non-mechanical counterparts, which is why there are just as many droid bounty hunters, torturers, and spies in Star Wars as anything else. These helpful machines aren't always so benevolent. Plenty of these droids are some of the most villainous characters in the whole franchise. There's a lot to be said for the potential of droids when it comes to merchandising as well, and there's nothing more Star Wars than that.
7 IG-88
A bounty hunter that has made brief appearances in several Star Wars properties, but he's best known for his appearance with all of the other bounty hunters on the deck of Darth Vader's star destroyer in Return of the Jedi. IG-88 has a history of bounty hunting that goes back to the Clone Wars, and his years of ruthless experience made him one of the Sith Lord's candidates.
At almost two meters tall and armed with a blaster rifle, he was built for intimidation as opposed to stealth. IG-88 did not have a master when he answered Vader's call but was working independently, unlike some other droids of the same make and model.
6 Mister Bones
The flimsy droids of the prequel trilogy were mostly mindless drones that could be blown over by a stiff wind. Mister Bones had gone through several changes, and the finished product was a savage and evil machine bent on death and destruction.
Built and modified by Temmin Wexley as a bodyguard when he was still a child, he kept the droid with him when he grew up to become a Republic fighter pilot. Mr. Bones was famous for how much he liked his work, often singing as he would attack, so it's fortunate he wasn't working for the Imperials.
5 EV-9D9
Normally protocol droids are ambivalent when it comes to their work, but EV-9D9 had a programming defect that made her enjoy torturing, dismembering, and melting down other droids. Her most famous appearance is in Return of the Jedi in which she's working in Jabba the Hutt's palace delegating tasks to the others.
After Jabba was killed by Luke Skywalker, EV-9D9 was reprogrammed and put to work as a bartender where she couldn't so nearly as many villainous deeds. She appears briefly in two episodes of The Mandalorian.
4 8D8
EV-9D9 didn't always work alone. Like many other protocol droids, she had a silent counterpart who carried out her sadistic orders and seemed to enjoy it, despite his unchanging face and total lack of expression.
8D8's original programming was as a simple smelter droid for mining or refining operations, but he was lobotomized reset to aid EV-9D9 in her systematic operation of dismemberment and torture, and he had a special talent for pain. After Jabba's demise, he was temporarily deactivated until being reprogrammed to work for Boba Fett.
3 K-2SO
A droid that started as a villain serving the Empire, K-2SO was reprogrammed to be an ally for the Rebel Alliance. Built for intimidation, strength, and power, these are some of the most dangerous droids in the galaxy, and having K-2SO on the Rebel side was one of the few advantages our heroes had.
Aside from being a fearsome opponent in a fight, K-2SO was also frequently used to infiltrate Imperial outposts and other installations. It was only because of this special talent that Rebel spies were able to steal the Death Star plans.
2 HK-47
A figure that recalls some of the galaxy's earliest history, HK-47 was constructed by Darth Revan to "burn holes through meatbags" as he so pragmatically states. He dates from just after the end of the Mandalorian Wars when Revan was looking for more precise and secretive ways to destroy his enemies.
HK-47 was programmed to search the galaxy for individuals or small groups that his master had deemed a threat and terminate them with extreme prejudice. After Revan was recaptured by the Jedi Order, however, the droid's automatic memory wipe protocol kept him from remembering his master. There were others, but he ended up killing most of them until being inadvertently reunited with Revan much later.
1 4-LOM
With a hideous appearance like this, 4-LOM would have to be a villain. This is intended to be a protocol droid, a type of 3PO to be specific, but that bug-like face might not be the best design for a throne room or fancy dress ball.
It was a common programming glitch that allowed this particular droid to break protocol and become a smuggler, thief, and bounty hunter. 4-LOM was one of the droids that appeared on the deck of the star destroyer with Darth Vader and IG-88. He was known for the cold, calculating way that he sought out and destroyed his prey, and during his career working for both the Empire and the Rebel Alliance.