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Originally posted by Turnbull:
I go along with Researcher and "power corrupts." I'd also say that the absolute net is, "Crime doesn't pay." Michael lost everything by the end of the Trilogy.
Everything except for his money wink .

I think that vengeance is the most important theme in the movie. The entire trilogy is based on vengeance, and shows that one act of revenge leads to another. Michael's obsession with getting revenge on his enemies just leads to more murders, and eventually, to the death of his own daughter in GFIII. He loses what was most important to him, his family, because he took revenge on everyone that ever harmed him.

"Fredo, you're my older brother, and I love you. But don't ever take sides with anyone against the family again. Ever."

"Fredo, you're nothing to me now; not a brother, not a friend. I don't want to know you or what you do."

"Only don't tell me you're innocent. Because it insults my intelligence."