I do not believe Fredo knew it would be a hit. I believe his speech in the boathouse. I don't think he was lying to Michael. The phone call he received after the attempt also reinforces this when he says "you guys lied to me". I believe he had a momentary feeling of self importance when Johnny Ola approached him about helping with the "negotiations".

Fredo may have been instrumental in getting Johnny Ola the meeting with Michael on the day of Anthony's party, and the hitmen were unknowlingly allowed on the property as part of Johnny Ola's entourage.

As for who killed the hitmen. If we look at the history of gangland slayings, we know that hitmen are often killed after a failed hit, sometimes even a successful one. You f-up...you get clipped. They were probably killed by a third higher-ranking hitman who was with them. Granted in real-life the bodies are not usually found at the scene of the hit, but that was just a good ole plot device.

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Senator, we're both a part of the same hypocrisy but never think it applies to my family.

...ya gotta get up close like this and BADA BING blow their brains all over your nice ivy league suit.