There is the classic slow motion gradual close-up shot of Michael,outlining his plan to kill Sol and Mac.
This is the moment when we realize that the old idealistic Michael is gone forever.
I do agree with Turnbull that Mike had choices,but only before he crossed the line.
Afterwards, as with any Mob Chief,there were killings that he had to do to protect his empire,
I will concede that he probably went too far with some i.e. Fredo,Roth.
As for the 5 families murders,I don't know many options he had.
Had he merely executed Tessio as a traitor,he still would have Barzini and his puppet Dons to deal with.
I do feel that Mike fell in love with the power and fear he generated,so it wasn't always "just business"
Even Tom,at one point asked Mike 'Do you have to wipe everybody out"?
Bottom line,IMHO, Michael was a bad seed who was psychologically pre-disposed to becoming the sociopath that he was.