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 wanted to kill everyone who opposed the Family.
It's alluded to in the film when after the Don comes home Sonny tells Tom to help him find "that old pimp Tattaglia". Tom demurs and sarcastically notes Sonny's growing reputation for violence.
I wondered if Sonny's character was at least somewhat inspired by Albert Anastasia or Lilo Galente, two bosses known more for violence than brains...