Michael's almost constant reference to Tom as Tom Hagen reflects what I've always contended is Michael's blaming Tom for the Corleone's misfortunes. But it begins even sooner when Michael introduces Tom to Kay as Tom Hagen and then says (needlessly) that Tom is not a Sicilian. At that point there is no blame, but it illustrates Michael's glandular discomfort with Tom.
Oli, I agree that Michael blamed some of the family's misfortunes on Tom. He said it all when he said, in the fishtank scene, "You're not a wartime consigliere, Tom--things could get rough." In the novel, after learning about Sonny's assassination, Tom himself realizes that he's "not a wartime consigliere--old Genco would have smelled a rat."
But, at Connie's wedding, Michael introduces him as "
my brother, Tom Hagen" [emphasis added]. That's the innocent, not-part-of-the-Corleone-crime-empire Michael talking. In II, after the Tahoe shooting, Michael (in the hyper-manipulative mode) says, "You're my brother, Tom." Poor Tom practically breaks down in tears and says, "I always wanted to be considered as a brother by you"--meaning that he always doubted that Michael considered him as a brother.
