He did some awful things to be sure. He studied and used the Dark Side, but he never seemed to fully give himself to it like Palpatine or some other Sith. I think its similar to VOTF when Luke and Mara were discussing why she wasn't ever corrupted, even though she did plenty of bad things, though Dooku's obviously a lot worse than Mara ever was. And i'm not sure if he believed he was the Chosen One, so much as he wanted to be and wanted others to acknowledge him as greater than he was. I know he had a particular dislike for Anakin as Yoda pointed out to him on Vjun.