Star Wars: Tales of the Jedi delves into the canon continuity’s Count Dooku and his gradual fall to the dark side, but it may also explain his involvement in the infamous Order 66’s programming. Order 66 is the macabre masterstroke of Palpatine’s plot to destroy the Jedi Order and replace the Republic with his autocratic Empire, compelling the otherwise altruistic Clone Troopers to assassinate their Jedi leaders (who they nearly always befriended) via brainwashing. The plotting of Order 66 and the brainwashing protocols of the Clones’ control chips may have been devised by Dooku, however.
In the third episode of Tales of the Jedi, “Choices,” Dooku and Mace Windu are investigating the death of Jedi Master Katri on Raxus Secundus. Dooku correctly hypothesizes that Katri was killed by a group of Raxan security guards, noting that the Jedi are not easily ambushed unless their assailants are people they trust. This perfectly aligns with the effectiveness of Order 66, which would understandably suggest that Dooku at least had a degree of influence on Order 66’s programming, though how aware he was of Palpatine’s whole plan is another matter.