There are huge pieces of evidence of this found especially in both the Book of Exodus and Deuteronomy, where God controlled individuals to be evil, so he could have a reason to harm them. In Exodus "he made Pharoah stubborn" and so on.
Originally Satan (Mastema) was called "ha-satan" and was a loyal servant, a spy to God and he would test the faith of individuals to see how much they believed and worshipped God.
So to say that the snake was Satan (when he is only mentioned 4 times in the Old Testament) would not be a correct thing to do. Please don't take this as an insult or as an attack. It is just something to think about.
But I would have to agree that the original sin would have to be both Adam and Eve eating from the Tree of Knowledge of Good and Evil.