I believe that the reason for Tom's calm and cool demeanor, which is almost a facade for the type of person that he really is, is that he was trained and schooled by Vito on how to keep his temper, to be reasonable, and when called for, cold and cunning. When you think about it out of all the children, Tom was probably with Vito more than any of them.
I think that he learned not to be offended and not to react in haste when offended or attacked. And at the same time he knew how and when to do what needed to be done, even if it meant having someone killed. Like Vito, Tom knew that he had to think things out before reacting.
Tom really knew the meaning of the term "It's not personal, it's strictly business."
Yes, Tom does garner sympathy from the viewer at times, as did Sonny and Vito. But he, just like them, was a stone cold murderer, despite the demeanor that he displays in most of both movies.
Don Cardi