DC, If you analyze the difference in the men, you would see what I meant.

Vito was a different man than Michael. He inspired love and loyalty. Michael, on the other hand, inspired fear and resentment. Vito never would have been in Michael's position because he never would have made a member of his family feel the way that Fredo did. His dealings with others was smarter. The death of Khartoum and his dealings with the bandleader show that he could be every bit as ruthless as Michael. He knew enough to only use that force when there were no alternatives. Michael got to the point where he didn't bother making that choice.

Sonny, on the other hand, was loud and angry. He was also a generous man who allowed his emotions to get the better of him at times. He could never have killed his brother. But again, I can't imagine him causing the resentment that Michael did.