Saruman looked into the eye of Sauron and saw an inevitable defeat, and fear of that led to corruption. In plain English: it was a sheer lack belief.
The chief form this would take with them would be impatience, leading to the desire to force others to their own good ends, and so inevitably at last to mere desire to make their own wills effective by any means. thus involved in the peril of the incarnate: the possibility of 'fall', of sin, if you will. In Letters of J.R.R Tolkien, Tolkien waxes philosophically about the involvement of wizards in Middle Earth, how they act as magical beings yet still have the moral malleability of humans. But if it still bugs you, maybe this explanation will help. For those who read The Hobbit (or listened to the audio book that annunciates weird, like 'Jay Ahr Ahr Tol-Keen') you'll remember that Saruman wasn't in the original novel, making his appearance here very debatable canon. It's a bit perplexing since Saruman is a stone cold baddie in Lord of the Rings, and many of you may be scratching your head, wondering what happened. If you've been watching The Hobbit movies you'll take note that we've been dealing with a kinder, gentler, friendly neighborhood Saruman.
