The most heartbreaking scene in Part II is the murder of Fredo, and the "Don's Birthday" scene following it. Mike had his brother killed, and yet Fredo was the only one who supported him when he joined the Marines, Fredo seemed very close to Mike at the wedding reception, where Mike introduces him to Kay. From a business standpoint, Mike made the right decision. Morally, I don't think he could have made a worse decision. Family comes first. Right when you hear the gunshot, and see Mike's head drop, you can tell right there he's thinking "oh sh*t, what have I done?" So heartbreaking...

And I agree with the lot of you. Cazale was brilliant in this film, and definitely should have been nominated for Best Supporting Actor. His untimely death was so unfortunate. He would easily be held in as high a regard as that of De Niro, Pacino, and Brando.