We exist to glorify God.
Of course that's His purpose, however if everything is pre-destined and there's no such thing as free will, why would God create those who don't glorify Him? Are all God's creation as he intended them to be or did God create a flawed product in mankind?
See, there's the rub. I believe our God is big enough that although He wants all his children to be in Heaven to worship him, He gave His creation free will for a reason and that is, we are to choose Him of our own accord.
Think about it this way: if everything was pre-destined for us, then what was the point in God creating Adam and Eve? Clearly He knew they would sin and clearly He had a plan for mankind's salvation as Jesus' birth, death and resurrection was foretold in the Old Testament long before Christ's birth.
So is God's creation of Adam and Eve imperfect if He knew (pre-destined) they would sin by eating from the tree of knowledge? As Jesus would be born thousands of years later and there was no path to salvation for Adam and Eve, where are they? Heaven or Hell? I don't pretend to know that answer.