Fredo would have done it again, and he would have done it intentionally. The depth of resentment, envy, jealousy--the sheer venom--of his outburst in the boathouse makes me think that he'd never be satisfied until his brother was eliminated.
Your question implies that Fredo might have been grateful to Michael for having been spared. Perhaps. But as Puzo said in the novel (in the context of Bonasera getting that call from Hagen re. Sonny's body), gratitude is the shortest-lived of human emotions. That's why Michael never gave an enemy a pass--and he was right.