Although my view may not be the most logical, I believe that Michael might have considered giving his brother a pass, until Fredo's boathouse outburst. That show of rage, resentment and envy meant that there was no way to let Fredo live.
I agree. Until that outburst, there was at least a possibility of Fredo just getting chased. When Michael saw how Fredo really felt about him, he had no choice.
"I got news for you. If it wasn't for the toilet, there would be no books." --- George Costanza.
