You might find
this interesting.
For those of you who don't want to read it, or don't have the time, this discussion of predestination versus free will was brought up. I quoted the Silmarillion where Iluvatar tells the Valar that no theme can be played but his and that even if they tried to go against His will, they would only become his instruments for things more beautiful. I pointed to that as saying that Eru predestined everything.
But later,
davem pointed out another quote that said that Men could shape their own lives apart from the Music. So it seems that Iluvatar has made a plan that everyone must follow, except men, who can do their own thing.
So what about Saruman. Perhaps, in the form of a man, he was able to choose for himself what he really wanted. He chose away from what Eru desired for him, and almost ruined all hope.
Yet you could make a case that Saruman's treachery was also part of the plan, and only 'made things more beautiful'. If he had not pushed against Theoden, then perhaps Rohan never would have come to Gondor's aid. So, indirectly, Saruman caused the winning of the battle before Minas Tirith, and incidently the victory over Sauron. But maybe that's stretching it.
