My feeling is that Fredo knew exactly what he was involving himself in. That was why he was so upset when he told Mike how he got involved- because he never thought he'd have to face his living brother. Sort of like Carlo. Michael knew all along that Carlo was involved in Sonny's death but it wasn't until the end that Carlo finally admitted it. And I think that is why Mike told Fredo that he was nothing to him anymore because he took sides against him. My feeling is that all evidence points to Fredo being invloved in the actual hit. As far as the hitmen go, my guess is that they were killed because "deadmen tell no tales."