This is always an interesting question. I think dt summarized it perfectly. Michael never gave an enemy a pass. He might have considered a pass for Fredo, but the wild resentment Fredo showed in the boathouse sealed his fate, if it hadn't been already.
MC, you're right about "I swear to God I didn't know it was gonna be a hit." Yes, indeed--what did Fredo think he was opening the drapes for? If--repeat, if--Michael hadn't decided to kill Fredo at that point, his statement could have been taken as proof that Fredo did know it was gonna be a hit, and was lying to save his life. Or, Michael could have written it off to Fredo's terminal stupidity. But, whatever: the outburst that followed sealed his fate.