It could have been a number of things.Maybe hiding out in Sicily for so long began hardening his heart.As he missed being in America.Then there was the death of Appolonia,which i believe contributed to his change.Also in the novel it says that he went through three years of the Dons tutelage,and like you said ,although the Dons wasn't as cold,Michael had had different things and aspects of his life from Vito's while beginning his "training".Which made him a lot more colder.Just some of my thoughts. Also Vito had never lost a wife also.
The hero and the coward both feel the same thing, but the hero uses his fear, projects it onto his opponent, while the coward runs. It's the same thing, fear, but it's what you do with it that matters. Cus D'Amato
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