Didnt Sonny want to kill all the heads of the families?
That was a good idea and an idea that Michael actually carried out. They never saw him coming.
They would have seen Sonny coming a mile away. But it was still a good idea, right?
Yes, per novel that was Sonny's idea, his master stroke. The other Family heads went underground and did not surface until after Sonny's death. It was a good idea but he didn't have quite the tools to pull it off.
Sonny's idea to kill all the heads of the families was a bad idea for its time. It was typical, hotheaded Sonny--predictable--the worst place for an acting Don to be in a time of war. And as we saw, Sonny's predictable reaction to Connie's second beating cost him his life.
Michael's killing of the other Dons was, as noted, not predictable. He made them think he was weak, then he struck. BUT, in the novel, Michael only whacks Barzini and Tattaglia among the Dons. He didn't have to whack the others--they saw the example he was setting and, presumably, got the message. I could make a case that, if Sonny had reacted by going after only Tattaglia, he might have split Cuneo and Stracchi away from Tat--they'd want to have a truce so they could go back to making money instead of hiding and fighting. That might have made Barzini show his hand. But it'd be too subtle for Sonny.
