Puzo and Copolla are more intelligent than to do anything on a superficial level. The choice Michael made was a very personal one. One that would forever link him to the family and cause him to be involved, forever an insider due to his actions. It was definatly a personal one. However I think there is a deeper psychological tool at work here. When you say "it's just business", that's a way of morally justifying your actions, not having to take blame for them. "They are the rules and we are playing by them", "I am taking orders", etc, these are all ways of not having to make hard moralistic distintions that otherwise may cause you to have a crisis of sorts when you realize you murdered someone.