He had to appease Sonny as well as resolve the issue in that particular instance. He still had to choose to insinuate himself into the plan, however, so your point still stands. I don't think Sonny would have accepted a bloodless plan. Perhaps Michael wouldn't have either, but at that point he may have been more willing to listen to Tom Hagen.

But yes, Michael sowed the seeds of his own destruction, and as the saying goes, pride comes before the fall. He thought he was too smart, too clever, too rich, too powerful to be have anyone out fox him.