I'm not sure that "tragic" is the right choice of words. Both Vito and Sonny were murderers in their own right. When you choose to live by the gun, it can't be considered "tragic" when you die by it. Maybe "sadder" is a better way to put in. And in that case I'd give the edge to Sonny because he was younger, he had young children, and his parents had to bury their son, which is the saddest thing in the world. To me, the only tragic death in the trilogy was Mary's. She was a total innocent, proving that the old adage, "we only kill each other," is a total crock of shit, accident or not.