[Suzerain of Sheol]

Well, yeah, he was just set up to fail. He believed he could learn all of Sauron's secrets and use them to overthrow him, and later, to establish himself as lord of the earth in Sauron's place.

I remember a scene in the Book of Lost Tales where the Witch-King comes calling at Isengard and they have a delightful exchange about who serve whom.

On the other hand, Olorin never wanted to go to Middle-Earth, and only sailed at the request of... I want to say Lorien (one of the Valar) but I can't recall, anyway, he was basically sent to be the voice of reason and temperance among the Istari.

Oh, and in case you're wondering, the other two wizards, Allatar and Pollando, both blue, went into the East and South, never to return. Tolkien mentions that they failed in their tasks, and it's likely that they gave rise to traditions of sorcery among the Easterlings and the Haradrim.

On the other hand, there's a theory that the Mouth of Sauron is one of them, corrupted by Sauron into his service and so twisted by evil that he forgot his own identity.