My vote goes for the look Michael gives to Neri indicating that it's time for Fredo to die. Why I say so is because Michael is so deceiving by making Fredo believe that he has been forgiven, only to be using that fake trust to make Fredo relax. Just the fact that he can win Fredo's trust while at the same time giving the order for Fredo to die is what makes it so chilling, IMHO.