There is no evidence to suggest Fredo knew it was going y be a hit. Fredo is portrayed as stupid and harmless. Let’s be honest what did murdering Fredo accomplish besides causing a ruftvin his immediate family(Kay and Anthony)and causing Michael to be haunted by it for the rest of his life. Doesn’t seem worth it especially considering even Mike said “he knows Toth misled him and he didn’t know they were foing to try and kill me”