In III he rationalizes it all to Kay. He says first his father was in trouble "so what am I gonna do?" Then the same rationale for doing what he did to "save" his children. I think he crossed the line forever when he had Fredo killed. In essence at that point he was "saving" the family by killing one of its members, traitor that he was.
"Io sono stanco, sono imbigliato, and I wan't everyone here to know, there ain't gonna be no trouble from me..Don Corleone..Cicc' a port!"
"I stood in the courtroom like a fool."
"I am Constanza: Lord of the idiots."
| 
