I actually think Brad Dourif's explanation fits better than he "felt" guilty. Looking at Grima's face he seems more awed at the army than regretting helping out Saruman.
Though in the long run the "guilty" argument works better, as we get to see (especially in the EE's) Saruman's mistreatment of Grima and his final "breaking" point. Looking over this scene, however, Grima seems more in awe and reverance to the size and "splendour" when seeing the army than anything else.
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Fenris Penguin
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