It may be cliche to point to the Godfather, but this scene is incredible. Throughout the entire movie, Michael is able to maintain his humanity; going to the theater with Kate, falling in love with the Italian girl, telling his dad about his son reading the funnies, etc. We see the murders and appreciate the ruthlessness of Michael when exacting revenge on the enemies of the family. But now we see the beginning of Michael's war on his own family and he loses his humanity and becomes a machine. A brilliant foreshadowing of when Michael later shuts the door on Kate's face in front of her children in Godfather II.