TB you make an excellent point, however Vito didn't kill Sonny because he made intemperate remarks and screewed around with too many women.

In thinking about it, Vito had to know in his heart of hearts that the only son he had who had the qualities to succeed him was Michael. Maybe thats why he kept after him to "have a talk" about his future. Vito also had to realize that Sonny would not be a prudent Don and that if and when Sonny took over he would run the family into the ground. Still he did nothing to remedy this. So maybe he was going soft, and maybe, as reprehensible as Michael's act was, it wass justified in the cirminal mind set which they all had.