I'd go so far as to say Michael is probably one of the best depictions of a pure sociopath ever put to film.

Everything to him is a deal. An arrangement. Other people are truly just players on his stage, nothing more or less. If they fail or succeed either way he will use it to HIS benefit, even if it comes at their expense. All human interactions he has after Sicily are business transactions. Even his interactions with his children in II seem almost calculated, like a man who is trying his best to act in a way society deems as fatherly - but even then there is a strange remoteness even to his small children, almost like a robot mimicking human behavior. The scene with Kay when she talks about their life doing some unspecified psychological damage to their son shows he doesn't care - "I don't wanna hear about it!" - it's more a wound to his pride that there might be something wrong with his son, than it is a matter of concern to him (whereas we see Vito is genuinely disstressed when seeing Fredo sickly as an infant).

I think every relationship in Michael's life - personal, romantic, or business - was sort of this highly calculated, highly manufactured facade, a mathematic equation, a transaction. Nothing more. Like he has no genuine interior world of emotion; no empathy. He is able to play easily on others' empathy (IE playing on Connie's sympathy for Fredo, and manipulating her into being unwitting bait for her own brother's execution) and exploit their human feelings for his own need. This is a man who hugs his brother warmly at their mother's wake, while giving the nod to Neri that the time has come to murder him. Even Vito, even in his coldest moments, was never so evil.

This is why in the original ending of 3, when Michael dies alone and broken, I don't feel sympathy as Francis wants us to. I think, good, you got what was coming to you, prick.