The emphasis of the Devil in the New Testament seems mostly to be a personal demon, ie it tempts a person and if the person falls into temptation, they create evil in the world and if they create enough evil, they go to hell when they die . Now Jesus was offered by the devil to be lord over the nations of the earth, which indicates that the devil could deliver on that promise, but that seems to be the only time where that is suggested

The demiurge on the other hand is a deeply malevolent, evil being which created and is in control over all material reality. It is not the ultimate Godhead, but it controls material reality. It’s goal is to trap God souls in material reality and then torture and destroy them; less of an emphasis on going to Hell after death, this world IS hell. One can see this because the universe is predatory; anything alive survives by killing and eating another live organism, which is just an awful concept. And in terms of humanity, the good suffer and the evil prosper. This explanation seems to me based on observation to be closer to the truth than the traditional understanding of the Devil

I guess both involve a spiritual struggle to retain goodness in the face of evil, which is maybe why the Godhead allowed this system, but the demiurge concept just makes evil way more in your face and all encompassing

(http://www.autoadmit.com/thread.php?thread_id=5213324&forum_id=2#45356278)