Michael knew that Sonny was a hothead, and would get himself killed sooner or later. Intellectually he couldn't have blamed Tom for Sonny's assassination. But Tom, as a lawyer and non-Sicilian, didn't take into account that Carlo, a Sicilian, would thirst for revenge, no matter how dangerous it was to him or how obvious was his involvement. So,emotionally, Michael may have held Tom accountable for the fakeout that led to Sonny's assassination. Another emotional factor: had Sonny lived, Michael might not have had to lead a life of crime. Michael might have been able to return from Sicily and lead a "normal" life. Michael may have resented that, and have held it against Tom.