Gatsby's downfall was this exact same thing - success through whatever means one deems necessary. He did not honestly earn the money, and he spent his life trying to cover up lies and make himself seem great. Daisy was the one part of his American dream that he did not yet have, but after Tom and Gatsby have their great argument in the city, Gatsby is sure he has just snagged her. However, Gatsby's penchant to lie about everything is his ultimate downfall. He lies about who was driving the car, which means that Tom is willing to tell Wilson that the car belongs to Gatsby. And just like that, Daisy escapes without punishment - and she still has a man - but Gatsby has lost everything.