Some excellent threads going at the mo.

Tom Hagen is one of the most interesting characters there is. Lots of screentime yet so underdeveloped. We know he is loyal to Vito to the end. To the absolute end, he loves him. He also loves Sonny, and thats where Mike's resentment comes in. The bond between Sonny and Tom probably made him jealous. He doesn't see Tom as his real brother, and when he assumed power, the first thing he did was demote him. He accuses him of not being a wartime consiglieri, but if anything, his calm coolness prevented the war from being a lot lot worse.

In part II Mike tells Tom that he kept things from him because he loved and protected him, but then proceeds to go back on this later in the film and humiliating him in front of Rocco and Neri...this to a man that actually ran the family for a few months. I think Tom reminds him of the days when he, HIS brother and HIS father were close knit and he was the kid brother having this life planned out for him.

The flashback scene is key i think, Mike HATES the fact that this guy, an adopted Irish kid, is telling him that he and his father have plans for his future, for me, that scene explains Mike dropping one on Tom time and time again.