[narfforc]

The very desire of the One Ring was enough to corrupt, and as is said: It is dangerous to study too deeply the arts of the enemy. Saruman would have crafted the ring at some stage after his corruption, and well into his sojourn in Middle-earth, and as for breaking the rules that had been set on The Istari, he had probably broken most of them by the time he had made his ring. That brings us to The Valar, they did not directly intercede against Sauron, so I presume they would not against Saruman, they knew how dangerous it was to incarnate Maiar into The Istari, if they had wanted to have kept them from turning, they should not have given them the weakness's of the body.