Janus, It's explained in the official novels of both AOTC and ROTS. He felt that the Jedi were wasting their powers, talents, and time serving an institution as corrupt as the Republic. Which is why he resigned.
He joined the Sith for personal gain, and political reasons. When Ki-Adi-Mundi said that "He's a political idealist, not a murderer." in AOTC, he was right about the political idealist part. Lord Tyrannus wanted a dictatorship in which only Humans ruled the galaxy, and his wish came true 20 years after his death.