I agree that Sonny made a bad Don. But I can find justification for some of his actions:
Whacking Bruno could be justified on the grounds that the Corleones needed to show that they were capable of quick, ruthless retaliation--not paralyzed by Vito's shooting. And Michael's involvement could be justified on the basis that no one else was capable of getting close enough to Sol to stop yet another attempt on Vito's life.
I continue to believe that a much better alternative would have been to leak info on McCluskey's corruption to the newspapermen on the family's payroll instead of shooting Mac, rather than after the shooting. Vito was a high-profile victim. Faced with blaring headlines about Mac's complicity with Sol (especially in removing the bodyguards from Vito's bedside), plus pressure from the politicians on the family payroll, the police commissioner would have no choice but to assign 24x7 police guards to Vito. He'd have to suspend or transfer Mac, and arrest Sol. That would have been the politically sophisticated solution. But it would have left us with no Godfather Trilogy.