Dooku had to kill a bunch of mandalorians and send the survivors into slavery when the jedi where sent to stop the mandalorian civilwar between the true mandalorians who are elite commandos and the deathwatch mandalorians who wanted to get back to there roots by becoming plunderers and pirates like there ancestors. that made him question the republic and the jedi. and that was when darth maul was killed and palpatine convinced him to turn to the darkside. shortly after that he got jango fett who was the leader of the true mandalorians freed to be the template for the clones.
Grevious was Kaleesh from the planet Kalee, how original lol. and he was in a shuttle accident.