Complex sentence: Fredo is returned to Nevada, where he privately explains himself to Michael: resentful at being passed over to head the family, he helped Roth in expectation of something in return, unaware, he claims, of the plot on Michael's life.
Split into simpler sentences: Fredo is returned to Nevada, where he privately explains himself to Michael. He was upset about being passed over to head the family, and helped Roth, thinking that there would be something in it for him, unaware, he swears, of their plans on Michael's life.