I think that Michael did not have any intention of murdering Fredo until the boathouse conversation. I think that Michael became enraged at Fredo's stupidity and weakness. I think Michael was almost apoplexic when hearing Fredo's words. The injury that Michael told Cardinal Lamberto about was an injury to Michael's sense of logic and order which required that Fredo accept Vito's placing him in a subordinate position, his being taken care of by Michael, and his lack of adequate intelligence, perception, and force.