A Probably too Sympathetic View of Gríma

[Encaitare]

When offered the chance of redemption by Frodo he does not take it.

He most likely would have accepted had Saruman not interfered and angered him. After killing Saruman, I suppose then, as you said, he figured they would not offer him further mercy.

We don’t know how old Grima is, nor do we know how long he has been Theoden’s adviser, but we could assume that it has been some time, and so the promise of Eowyn might not have been made from the outset.

The thought I have on this is that maybe Saruman's "buying" Grima was a gradual thing. At first he might have just asked for some seemingly harmless information about Rohan, then maybe for some info about its military status. Grima was probably serving Theoden for a while until the time Saruman finally flat-out struck whatever kind of bargain they made, but Saruman already had one foot in the door.