Both, the Bible and The Divine Comedy, depict Satan as another prisoner suffering his punishment in hell. Why, then, do we think that Satan is the ruler of hell, and all his devilish minions will torture us for eternity?
Is this an appropriate sub for this? Neither Satan nor Hell are historical figures. Can we narrow this down by defining who the question refers to (i.e., who are the "we" who believe Satan to be Hell's ruler)? What period are we talking about?