Why Does Hell Exist?

Hell, as described by Dante in The Divine Comedy, is filled with people being tortured forever by demons. Why does God allow the demons to torture people forever?

Hell, as described by C. S. Lewis in The Great Divorce, is filled with people who decided not to be with God. Why does God allow the non-believers to continue existing, forever miserable in their separateness from Him? After a person chooses Hell, God could erase them from existence. Why doesn’t he?