When Michael is at Don Tomassino's coffin he vows on the lives of his children to sin no more, and at once he turns everything over to Vincent, who in turn orders the murders of Luchese, the Swiss Banker f*ck and the archbishop, and of course Connie gives Altobello that almond flavored canolli.
Michael had to know this was going on, and he was called on the oath he tok because by "sinning once more" it cost Mary her life.