General Grievous from Star Wars is a pretty amazing villain based on his daunting appearance alone. He is a giant, altered alien with four arms who wields a number of lightsabers without actually having the force. His appearance in Revenge of the Sith was pretty sudden to fans, as his character wasn't fully explored until later in The Clone Wars television series.
Grievous may become a bit more of a "comic-relief" villain during the animated series, as he is often seen arguing with droids and being bested by the Jedi, but that doesn't mean that he is also totally brutal. In fact, he may be one of the most devasting villains in the Star Wars universe when analyzed.
He Was A Jedi Killer
It is no secret that General Grievous was a Jedi Killer. Though it is never outrightly said in The Clone Wars, it is constantly hinted at that Grievous is constantly on the hunt for Jedi and has zero hesitation when it comes to killing them.
In the 2003 iteration of Clone Wars created by Genndy Tartakovsky, Grievous goes up against a number of Jedi all at once, including Ki-Adi Mundi, Aayla Secura, and Shaak Ti, and actually was the one who killed the latter. In a deleted scene of Revenge of the Sith, he still is the character who causes her demise.
Took Master Neebo's Saber
Grievous liked to collect Jedi lightsabers and other weapons. He considered them trophies in his collection, and he would then use them to face off against other Jedi in a horror show of dominance.
One of the sabers he took during The Clone Wars, belonged to Jedi Master Neebo. Neebo was actually never seen facing off against Grievous and actually went missing. Jedi Master Kit Fisto discovered it in Grievous's possession and knew of her demise due to the fact that he had been a known Jedi killer.
Used Master Sifo-Dyas's Blood
What is interesting about Grievous having Jedi Knight Sifo-Dyas's blood running through his augmented veins is their shared connections to Darth Tyrannus, Count Dooku. Grievous works under Dooku and was trained to fight with sabers under his eye.
Sifo-Dyas was also Dooku's closest friend growing up. In Dooku: Jedi Lost, the story dives into the character's backstory and how he and Sifo-Dyas grew up with one another, how Sifo-Dyas was the only one Dooku actually trusted in his age group. To have his blood running through the man he trained, who he considered to be an abomination, is quite interesting.
Murdered Every Force Sensitive He Came Across
General Grievous doesn't take any short cuts and he is ruthlessly bloodthirsty. He doesn't let any force user who comes into his path live if he can stop it—and that means anyone, not just the Jedi. He would kill a battalion of younglings if he had to, with no remorse, just like Anakin. He would often go seeking out those who were fForce sensitive so that he could kill them before they discovered more of their abilities.
Went After Younglings
Speaking of Jedi younglings, there was actually an entire arc in The Clone Wars where Ahsoka helps take a group of them to construct their lightsabers but gets invaded by Hondo's gang of pirates.
General Grievous sees this as the perfect time to collect new trophies and kill young Jedi before they can evolve further. He actively tries to murder them in the most nightmarish of ways resembling The Exorcist, but is halted by Ahsoka who defends the younglings.
Commits Genocide
This is a pretty big one. In order to punish his former apprentice for leaving his side and trying to murder him, Dooku decides to destroy what Asajj Ventress holds most dearly, her family. Upon Dooku's orders, an all-out war ensues between General Grievous and the Separatist droid army against the Dathomir Nightsisters.
Grievous ambushes and overruns the witches completely, leaving their fortress in total ruin after the sheer massacre. In Jedi: Fallen Order, Merrin says that almost all of her sisters were killed, leading to the genocide of the Nightsisters.
Kills Mother Talzin
Grievous doesn't stop at the total annihilation of the Nightsisters. In the following comics of the Clone Wars, Grievous decides to continue the hunt for the head of the Nightsisters, Mother Talzin. Talzin was the biological mother to Maul and Savage Opress and greatly cared for the Nightbrothers and Sisters who had followed her. It seemed like the only one she had left was her only remaining son, as she was the only one left for Maul, as well.
Yet, Grievous and Maul didn't get along. The Jedi killer used Talzin to bring Maul out of hiding and save Dooku from their clutches. Maul and Talzin fought side by side with one another as the Mother defended her son. Palpatine and Talzin also had an interesting dynamic, as she almost became his apprentice instead of Maul and Dooku. But it didn't matter; Grievous killed Talzin right in front of Maul as he was retreating, forced to watch his mother turn to stone.
Murdered General Tarpals
General Tarpals was a Gungan who fans of the Star Wars universe actually liked. In a battle on Naboo, Tarpals got into a battle against Grievous and was able to badly wound him, impaling him through the chest.
Unfortunately, however, Tarpals was also mortally wounded by the cyborg. His sacrifice was heavy, but it lead to the capture of Grievous, though that was eventually undone when Padmé was forced to trade him for the captured Anakin Skywalker.
Killed Nahdar Vebb With A Blaster
Nahdar Vebb was to Kit Fisto what Anakin Skywalker was to Obi-Wan Kenobi. Kit Fisto was the young Jedi Knight's master, and the two often accompanied each other on missions across the galaxy. One of these missions led the amphibious duo to face off against General Grievous.
Like Anakin, Vebb seemed to lack patience and seemed to hold a lot of anger when it came to the cyborg. The three got into a heated battle saber against saber, but, in a dirty move, Grievous ended up shooting the young Jedi Knight down with a blaster.
Tortured Eeth Koth
Eeth Koth was an Iridonian Zebrak Jedi Master who was captured by Grievous and then brutally tortured after being ambushed on his flagship. Grievous recorded himself torturing the young Jedi and sent it to the Council as a means to hurt them. They all seemed relatively affected by the pain their fellow Jedi Master was enduring.
He also then promised that he would kill Eeth Koth and go after all the other Jedi to do the same. However, Koth wasn't actually killed until after Order 66, when his daughter was chosen to be Darth Vader's apprentice. When Koth refused, Vader murders him and takes the child.