[Holbytlass]

Saruman stepped onto Middle-Earth on the side of good, head of the white council, follower of Eru's plan. Somewhere along the way, he wanted to be mightiest, strongest, most powerful being, which doesn't always equate to evil. I don't even see Saruman in the beginning wanting to be evil just most important. Unfortunately, at the time, the thing that could make this happen for him is the One Ring, which is evil.
In Saruman's wish for the ring, he unwisely uses the palantir to see what Sauron is up to, but that entraps himself to Sauron. And in this entrapment, both have some means to their own ends. Saruman might not like being under Sauron's thumb but at least he can use this position to his own advantage by gaining information from Sauron. And Saruman is biding his time until he can recover the ring for himself. Likewise, Sauron thinks he has an 'insider' to the white council and to his enemies.
Saruman got in over his head, and we know what happened to him because things didn't go according to his plan.

Is Saruman for or against Sauron? Saruman is for himself, but his choices cause him to be with Sauron and then under Sauron. Sauron is just as good a 'sweet talker' as Saruman, probably better, which puts enough thoughts into Saruman's head that if he joins Sauron, he could be in a better position to become mightiest, after his betrayal.