The Treason of Isengard: Eru's Plan?

The simple explanation might well be that in ME everything happened for a reason, the reason being Eru's will. Consider the Ring: Bilbo was meant to find it, and Frodo was meant to be the one chosen to destroy it. Hence why not extend this further by saying Saruman was meant to become a Traitor, ultimately to the ruin of Isengard AND Mordor? The problem I have with that is that it would be cruel of Eru to plan the Downfall of Saruman like this, that is unless the plan was made after Saruman turned to evil ways whilst he was still part of the Council, even before Bilbo found the Ring.

Or is that just too simple a way to look at it? The posts on this topic are getting very complicated, so a more simple way of explaining things is given here.