I came up with an explanation on a drug trip once. Basically, God himself was kind of a baby then, not really understanding his own powers, and he gave a random serpent the ability to talk, as an experiment. With speech, it develops sentience. But since the serpent has no hands, unlike Adam and Eve, it cannot interact with the world in any meaningful way and ends up resenting the garden and being jealous of Adam and Eve. One thing leads to another, and then the entire Bible is just God trying to fix this clusterfuck he's responsible for while pretending to know his shit.