[Altariel]

Perhaps Grima's pent-up anger and loathing for his abusive master had something to do with it... you never know.

And I think, really, it's not a question of whether or not the hobbits' bows could have killed Saruman; they were probably too terrified of him to act in any case. Also. Saruman had the power of his voice, still, and he probably used that to hold sway over the hobbits...