The thing about Tom is it took him a long time to get on board with Michael's plans. He truly was not a wartime consigliere and lacked the "Sicialian cunning" necessary to ferret out enemies before they do harm. Some on these boards blame him for Sonny's death (I don't), and everyone agrees that his first instinct is always to patch things up.

But when he's with the program, he is tough as nails. We see this in GF when Tessio asks if he will intervene and get him "off the hook." Tom coldly says "No chance Sallie," and then gestures with his head indicating the hitter should get on with it. We see this toughness again when he ruthlessly compromises Geary. By the time of Mama's funeral he is truly disgusted with Fredo and his betrayal, and again he goes out of his way to be cold, telling Fredo Mike is out of the room waiting "for you to leave."