[Thinlómien]

I'm quite skeptical about spells in Gríma's case - I don't think he was under Saruman's spell. That thought just makes him a victim too. (In a way he was - but not this way.)
Really, Gríma served Saruman because of his own will and later because he had no other choice really.
This is the way I understand him; he became "evil" because of his human greed and desire for power. He's a human character after all. A victim of his weakness. I pity him as I pity Gollum.