[Zigûr]

On some level Saruman believed that Sauron could be manipulated towards a positive purpose, or at least what he thought was a positive purpose - the ordering of Middle-earth.

"...the Wise, such as you and I, may with patience come at last to direct its courses, to control it. We can bide our time, we can keep our thoughts in our hearts, deploring maybe evils done by the way, but approving the high and ultimate purpose: Knowledge, Rule, Order..."

He evidently believed the same of the One Ring:

"Why not? The Ruling Ring? If we could command that, then the power would pass to us."

I think Saruman had lost his moral compass, as it were. He no longer perceived evil as a thing in itself, which he must have originally known it to be, but rather a matter of perspective. I would argue that in his impatience he had convinced himself that evil was relative: Sauron could be turned to good, or if not him, then the Ring itself. Failing that, the concept of what 'good' was could be changed by someone sufficiently powerful: an übermensch, if you will.

We might imagine the toil of the Wizards. They have been engaged in endless labour in Middle-earth for two thousand years, confined in the bodies of Men. Sauron was only growing stronger, not weaker. It must have been difficult for Saruman to maintain his conviction that their mission was worthwhile, especially given that their memories of the Valar and the West were clouded and vague. Gandalf retained his faith. Saruman did not, and came instead to admire and envy Sauron for his power and mastery.

I think from that point of view it's possible to understand how Saruman came to no longer view the Ring as evil or the abandonment of his mission as a moral failure. Compare Sauron's view after the Downfall of Númenor: in his view it was the Valar who had failed and he who had stayed true. But to him this did not mean enacting the will of Eru, but rather achieving his own personal goal of ordering and perfecting the world. In emulation of Sauron, Saruman was evidently approaching the same system of values:

"There is no hope left in Elves or dying Númenor."

The rejection of the Children of Eru, and in particular the blessed among them, reveals to us a deeper abandonment of spiritual conviction. Good and evil were only meaningful concepts in reference to a higher power that Saruman no longer had any faith in. In this frame of mind I can understand the idea that Saruman would reject the concept of intrinsic evil, thus coming to believe that the Ring was not fundamentally dangerous for its wielder, nor that he himself had become corrupt.