Don Lights, everything you said is true. But that doesn't preclude the possibility that Michael felt Tom was to blame for not being more vigilant. In the novel, Hagen blames himself for Sonny's death thus: "He was, he knew, no fit consigliere for a family at war. He had been fooled, faked out, by the Five Families and their seeming timidity...Old Genco Abbandando would never have fallen for it, he would have smelled a rat..." Perhaps Michael resented the fact that Sonny's death thrust him into the line of succession--had Sonny lived, Michael might have been able to return from Sicily to a normal life.

I personally blame Sonny's hot temper, not Hagen, for Sonny's death. But, as a non-Sicilian, Hagen failed to appreciate that Sonny's public humiliation of Carlo would fill Sicilian Carlo with an unquenchable need for vengeance. Add to that Carlo's ongoing resentment of being only peripherally involved in the family business, and it should have led to Hagen thinking of the possibility of betrayal.