Dooku needs a replacement for Jango Fett, who lost his head at the start of the Clone Wars. Boba is out there somewhere, but he is a wee bit young. So, Dooku goes to Rattatak, a back water planet with no-holds barred combat. On that planet, a lone survivor of the Sith race (don't ask me how the hell she lived for 1000 years) has participated in this free-for-all, and has come to the gladatorial games held for Count Dooku's benefit to win a place as his new right hand man.
Wait, you say. Isn't Durge, the last survivor yadda yadda yadda bounty hunter yadda yadda ... yeah yeah yeah. Dooku figures one bounty hunter was good, so two working for him is better.
The point is that Dooku recruits a lot of people who feel that their species has been wronged in past wars, or that their family was wronged, or that they were wronged. He feeds off of vengeance, of saying he has an easy way to right the wrongs of the past. The evil of the Separatists is the evil of Al Qaeda, shifting the world balance by any means necessary. That sets the stage for the Republic to mutate into Empire ... maintaining the status quo by any means necessary.
And the Cantina band played on ...