I always thought (In the movie) Michaels's reply is almost making fun of the "It's strictly business" axiom always said to him...at that point.
Like of course it's personal. Michael see's the hypocrisy, even in his brother Santino taking it personal (Arguing with Tom) Sonny mocks Michael a little bit, and being hypocritical himself.
So Michael's reply is matching his brothers' tone back to him.
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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