Saruman's Treachery
 

At first I thought that Saruman had only betrayed Gandalf, and the rest of M-E, and turned to Sauron, but it seems that he also turned against Sauron too. Was he working on his own against both Sauron AND the good guys, or was he partially allied with Sauron?

Saruman had first betrayed the West by turning to Sauron secretly. Later, when Saruman became aware that Gandalf knew the location of the One Ring and that it was in possession of a Hobbit, he began to act to seize the Ring himself. Thus he tricked Radagast into luring Gandalf to Orthanc in a bid to "convert" Gandalf to his camp. Some suggest that the crows which attempted to locate the Fellowship in Hollin were sent by Saruman, not Sauron. Saruman's power play was devious and he, at all times, attempted to position himself so that if Sauron recovered the Ring, Saruman would have appeared to be a loyal servant. Only after Merry and Pippin were captured did he act overtly in his own interest. So he was twice a traitor, first to the West, then again to his new master -- Sauron.

Perhaps from the start Saruman didn't intend to side with anyone, as he wanted power (and the Ring as the incarnation of power) for himself. But while using the Palantir of Orthanc he got in too close a contact with Sauron and got trapped by him. Thus some of his actions were inspired and guided by Sauron, though I don't think Saruman was completely overpowered (the way Denethor was on similar occasion). I don't believe he ever really wilingly sided with Sauron, only for his own convenience or under control; I think it was ultimate power he was after.
But he definitely betrayed the White Council and the case he had been sent to assist.

In UT, it talks about how Saruman decieves the Nazgul when they are hunting for the ring. He was clearly looking to play both sides to his advantage for some time before the war of the ring. I also read that that was why he agreed to attack Dol Guldor... to get Sauron away from where the ring might be.