I think that the Vito you see is more of the cinematic creation. If you read the book, he didn't kill Fanucci out of desperation or anger. He killed him because that was the first step in becoming the man he was destined to become - The Godfather. All his life, he was underestimated. He was thought to be weak and sick and stupid by many because he kept things inside. Yes, he had a tragic life, but he wanted that rise to power. He wanted to be the one who held the strings. The circumstances of his job loss, Fanucci, and meeting Clemenza helped to get him there. Realistically, Clemenza and Tessio are the ones with the criminal experience. Why does Vito become the one to talk to Fanucci? And remember the scene at Vito's house? How he tells Tessio and Clemenza that, if he gets Fanucci to take less money, they will owe him? He is a born leader, a natural force, and the quiet, peaceful man was the fake Vito.

That said, in the book, he dies saying how life is so beautiful. I don't believe that is a man who was unhappy with his destiny.