At face value you could see the irony in Vito's statement about massacring his boy. You live by the sword, you die by the sword. More expected than ironic. But Vito exposes his fatherly love before the scene in Bonesara's funeral home. After Tom tells him Sonny is dead Vito says, this war stops now. Kind of ironic that it takes the death of his own son to stop the war.