I believe that is the saddest part of any movie. The way he screamed shocked the audience as it shocked even Kay who couldn't believe he cared about Mary that much.

"Well you just do what I tell you to do! Goddamn it! If I had a wartime consiglieri -- a
Sicilian -- I wouldn't be in this shape! Pop had Genco -- look what I got." - Sonny Corleone