It does make me wonder though... Michael saw Sonny in action as the acting Don... he was concerned.... he saw that Sonny was a bad Don as Vito said later.
I don't think Michael ever thought Santino a bad Don, per se. I never really thought Michael was concerned about the fate of the Family...after all, his brother managed to get Michael out of the country and out of harm's way (the tragedy that befell Michael in Sicily only occurred
after Santino's demise). Santino may or may not have been able to "get Sollozzo" without getting the Cop, but Michael figured it out first (in the book, the narrative said that Santino's biggest mistake was sending Michael to kill Sollozzo and McCluskey).
Santino was no diplomat, and in that kind of occupation, you need to be. Michael was, as was his father before him.
