I think interpretations change. People say that the snake was the devil, but the Bible never says that. All it says is that he was a serpent. Actually, most people seem to ignore the fact that the serpent's punishment was him being turned into a snake.

Genesis 3:14 "And the LORD God said unto the serpent, Because thou hast done this, thou art cursed above all cattle, and above every beast of the field; upon thy belly shalt thou go, and dust shalt thou eat all the days of thy life"

If the serpent was a snake to begin with, then why would his punishment be "upon thy belly shalt thou go, and dust shalt thou eat all the days of thy life"?

Prior to monotheism, gods weren't always seen as good. Greco-Roman, Norse, Egyptian... all of those ancient polytheistic religions are prime examples of that. Judaism/Christianity is clearly inspired by many of the ancient religions and the amount of inspiration varies between the writers. The Bible is a bit confusing because of that. Mainly because it's out of order based on which parts were written first. Genesis isn't the oldest book in the Bible. When put in order, you can get a better idea of the evolution of that religion.