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.
Well I disagree somewhat...
There's novel Sonny and movie Sonny.
In the novel Sonny does not consider the murders of all the other Family heads until the Corleone Family position becomes dangerously precarious. The Family is somewhat overmatched. The Don's political network is neutralized; Tom is not quite a wartime consigliere; Tessio has mellowed and Clemenza lacks "youthful energy". Sonny had wanted to fight a holding action until his father could rejoin the fray but events on the ground made that impossible. Sonny was nearing a point where he had to either "go long or go home".
Eliminating the other Family bosses could probably end the war but at the very least it would definitely buy the Corleones some breathing space until the Don recovers.
That down time would allow more businesses to get up and running and reduce the likelihood of internal dissension/treachery.
In the film I believe the only time Sonny is shown even considering such a move is when he's yelling at Tom that he will end the stalemate by killing Tattaglia (and the others?) It's unclear if he means everyone. In the beginning he also states that he doesn't care if he has to fight all the other Families.
But his primary ire seems to be at Tattaglia. If Sonny had been able to get to Tattaglia and Barzini he may well have given Tom permission to set up peace talks with the remaining bosses.
But since Sonny thought of it (novel), Vito planned it (film) and Michael actually carried out the murders of all the other bosses I think that Sonny was not really that different than his relatives. His caginess and analytical skills definitely weren't on Michael or Vito's level but all three Corleones had the same instincts when it came to dealing with their enemies.
