Up until Saruman imprisoned Gandalf in Orthanc, everyone (except Sauron, I guess) seems to have believed that Saruman was on the side of good - while his influence was growing in Rohan. It seems like this situation, where he was a known ally, head of the White Council, and acknowledged greatest expert on the Rings -- but secretly had his influence working in Rohan -- would give him a much better chance of getting the Ring himself than being the enemy of both Gandalf and Rohan.
So why did he ask Gandalf to turn evil, vs. pretending to still be good and (once Gandalf asked him about Bilbo/Frodo's Ring) setting up a version of the 'Council of Elrond' where he could name himself as one of those responsible for destroying the ring, thus allowing him to claim it himself?
Did he really believe Gandalf would go over to his side?
Was it just that he didn't think he could hide his Orcs?
Or was it due to Sauron's influence on him through the Palantir making him less rational?