[Amanaduial the archer]

It could be that Grima, like all men, was simply weak, and that the power of trhe ring was too much for him - the temptation of everything it could promise, especially when times were hard and uncertain for men. Another arguement in Grima's favour could be that he had seen Saruman's power at work on Theoden - he may have been in fear of it, as he had seen it at very close quarters, and so was in no doubt of what it could do and to what extent.

However, I don't think this is the whole story: mainly, Grima was probably just out for what he could get - and he didn't really care for what got in his way. I don't think this can merely be attributed to the 'weakness of men' which is mentioned so often by the elves - this is something much closer to home. Mainly, I think Grima was just too selfish - unlike the other Men in the story, he didn't resist the ring, he was working with it. But when things stopped going his way, he pleaded that he had merely been under it's power, asking that the Kings would take him in - basically, he would have changed his allegiance in a second. He was always, it seems to me, just out for what he could get.