Count Dooku / Darth Tyranus

Tales of the Jedi shows Dooku decades before he became a villain. He's an elegant, demanding Jedi Master with a young Qui-Gon Jinn as his apprentice. On a mission to rescue a senator's son, he sees the Republic protect the powerful and ignore the suffering of ordinary people, and he doesn't forget it.

Later, he and Mace Windu investigate the death of Master Katri on Raxus Secundus, against the Council's wishes. When Mace gets a seat on the Jedi Council and Dooku doesn't, his distrust of the Order and the Senate only grows.

Still a Jedi, and already following Darth Sidious's plan, Dooku enters the Archives and erases the records of the planet Kamino using Master Sifo-Dyas's security code. Then Qui-Gon's death on Naboo shakes him, and he goes to demand answers from Sidious. Master Yaddle discovers the connection and tries to bring him back; he kills her in a duel, with Sidious watching.

The clone army started with a request from Sifo-Dyas to the Kaminoans, but it was Dooku, under the Sith name Darth Tyranus, who chose the genetic template. In Attack of the Clones, Jango Fett tells Obi-Wan he was recruited by a man called Tyranus. At the end of the film, Sidious greets Dooku as Lord Tyranus, and the audience realizes who was behind the order.

In other words: the same man who commands the Separatists built the army that will fight them. Both sides of the war have been controlled by the Sith from the start.

Dooku appears on Geonosis as head of the Confederacy of Independent Systems. He visits the captive Obi-Wan and tells him a half-truth: the Senate is under the control of a Sith Lord named Darth Sidious, and Obi-Wan should join him to bring that Sith down. Obi-Wan refuses, citing Qui-Gon.

When the Jedi and the clones arrive, Dooku flees. In a hangar, he defeats Obi-Wan and cuts off Anakin's arm. Yoda takes him on and deflects his lightning with his hands. Dooku escapes by dropping a column on the wounded and takes plans for a superweapon to Coruscant.

During the Clone Wars, Dooku runs the Separatist war effort and prefers intrigue to the battlefield. In the 2008 animated film, he sets up the kidnapping of Jabba the Hutt's son to frame the Jedi and win the Hutt trade routes for the Separatists. The plan fails thanks to Anakin and newcomer Ahsoka.

He also secretly trains the assassin Asajj Ventress. When Sidious grows wary of her power, he orders Dooku to get rid of her, and Dooku obeys by having his apprentice's ship destroyed. Ventress survives and returns to Dathomir seeking revenge. The Nightsisters offer Dooku a new apprentice, Savage Opress, who was actually groomed to kill him.

When the Jedi find Sifo-Dyas's lightsaber, the investigation leads to the Pykes, who confess they were paid by Tyranus to shoot down the Jedi's ship. Dooku eliminates Silman, a survivor of the attack, before he can tell what he knows. The Jedi learn that a Sith Lord was tied to the clone army from the very beginning, but they don't walk away from the war.

In Revenge of the Sith, Dooku believes he's staging the kidnapping of Chancellor Palpatine. He takes down Obi-Wan and faces Anakin, who cuts off both his hands. Palpatine tells Anakin to kill him. Dooku looks at his master and realizes, too late, that he was just the piece Sidious needed to sacrifice to gain a younger apprentice. Anakin beheads him.

The man who left the Jedi because he thought the Order was corrupt dies having gained nothing for himself. The Separatists he led are wiped out soon after, on the orders of the same master.