I don't think Sonny thought he was a bad Don. I think he viewed himself as a different kind of Don. His father and brother were both smart, I think he knew he would never be on their level of intelligence. I think he felt that he would be able to compensate for that with his "strength". He wanted to go to war, he didn't want to sit and think out what the next plan should be, he thought he could just beat them rater than out think them, and to him that didn't make him any better or worse, just different than his father and brother.