Why is satan illustrated as Gods?

friend or working for God. All throughout the OT the devil does Gods work. Satan is used many times by God to rise against nations in those times. Not to mention the wager between Job. Satan was never an angel in heaven. There are passages that christians think refers to him but they actually refer to the kings who were in control at that time. Anyways, why satan, later on in the NT actually is now seen as to now being against God fully?