Warning! This post contains SPOILERS for Star Wars: Tales of the JediDespite his corruption to the dark side by Darth Sidious, one moment in Tales of the Jedi might have saved Dooku long before he gave up on the Jedi Council and the Republic in the Star Wars canon. Having become disenfranchised by the Jedi Order's problems such as their hubris and complicity in the corruption of the Galactic Republic, Dooku became a voice of frequent contradiction that was often ignored. However, the Jedi could have saved Dooku from the dark side of the Force had they made a different decision following his mission with Mace Windu seen in Tales of the Jedi episode 3.
Following the death of Jedi Master Katri, Dooku and Mace Windu were instructed to recover her body. However, Dooku sought to exceed their mandate and investigate the reasons behind her death, an act which resulted in conflict with a group seeking to fight the corruption of their world's senator while seeing the Jedi as nothing more than "lapdogs of the Senate". Not only did Dooku sympathize with these fighters, but he was also displeased to learn that Mace Windu was given the open Jedi Council seat upon their return to Coruscant. Had the Jedi given the seat to Dooku and started listening to him, the odds are strong that his seduction to the dark side could have been prevented (along with the entirety of the Clone Wars).
Dooku Joining The Dark Side Was Inevitable - Because The Jedi Wouldn't Change
The mission and its results seen in Tales of the Jedi episode 3 serve as the perfect microcosm of Dooku's conviction and the deep frustrations he had with the Jedi Council. The Council wasn't interested in learning why Katri died, and Mace Windu was perfectly happy to simply follow the rules and collect her body for burial at the Jedi Temple. Even when Dooku learned the murder was rooted in resistance to Republic corruption and the belief that the Jedi only fought for the rich and powerful, no action was taken. Instead, the Council gave Windu the open seat for following the rules while Dooku's actions were seen as problematic, having led to the death of a senator (and largely ignoring the fact that he was corrupt).
The Jedi's refusal to change resulted in their own downfall, though Dooku did try to motivate reform. At one point in the Star Wars canon, he had honestly tried to change the Jedi Order from within by seeking out a Jedi Council seat himself. However, his voice was rarely listened to due to the Jedi's pride, believing they were infallible as servants of the light which ultimately led to Dooku leaving the Jedi Order and eventually serving Palpatine. Had Dooku been given the open seat instead of Mace (and actually been listened to), the Jedi Council might have saved Dooku from the dark side while also saving the Order in the process.
Star Wars Wants You To Think Dooku Was Right (In A Way)
While Dooku's methodology and decision to embrace the dark side were wrong, his beliefs were justified and sound. As seen in Tales of the Jedi, the Council had allowed the Order to become far too close to the Republic, so much so that they began keeping the peace as defined by the galaxy's most dominant (and corrupt) government. Had the Jedi Council not been so prideful, Dooku could have helped them change on a level that might have restored the entire Jedi Order itself, rather than bringing about their end using the dark side and the manipulations of Palpatine in the Star Wars galaxy.