What follows is a response I had sent to a Christian friend who has friends and family members facing cancer and thinking through the issue of God's will and cancer.
Response: the questions you raise about cancer and God's will are of course very relevant. for years I would have somehow believed/thought that things like cancer (or any other horrible thing) were somehow "allowed" by God in some mysterious way as part of His sovereignty. I know "allowed" is a loaded term, but I know that when Evangelicals use that term, it is stronger than it's absolute, philosophical meaning. Of course by definition everything that happens must somehow be "allowed" by God because He created everything including the free wills that both humans and angels possess. Most of my thinking as an Evangelical would have put the term "allowed" in almost a Job-ian sense. Satan checked with God, God said "O.K." and Job gets afflicted. God allowed it, but didn't directly cause it. I think the Calvinist/Augustinian perspective, albeit cruel and to me evil, is actually more intellectually honest - they don't bother with trying to water down terminology. God causes everything because He is in meticulous control (omni-control) of everything that happens. They interpret any and every passage that somehow contradicts their notion of sovereignty as essentially meaningless. Thank God no Christian believed that before St. Augustine! Only the Gnostic heretics believed that before the 4th century. If only Piper knew that!
I am personally of the perspective that many things happen in this world that ARE NOT God's will. In that sense, He isn't "allowing" them for some purpose (not that He can't bring great good out of each and every tragedy). When an child is abducted, raped and horribly tortured to death (to take the worst possible scenario I can conjure up), very few would appeal to God's will, God's allowance, or some "divine purpose" being fulfilled in such an event happening. In other words, when it comes to that type of horrific evil, no right-thinking Christian would puts God's authorship on it, no matter how indirectly (i.e. "allowing it for a purpose"). We know better than that. It would be a blasphemous thought. I put cancer in the same category. It is an evil disease that tortures people to death. I've lost far too many friends at this point to this wicked enemy to ever put God into an equation with the word 'cancer' in it. Does that mean we always see it healed? No. And yet, I believe that it is always God's will to destroy cancer and heal it. The fact that His will is not always done on earth "as it is in heaven" is precisely why Jesus commande us to pray for it. There are many other factors involved in this fallen world - beyond the factor of God's will. This is the world He created. A world where crap can and does happen and He has NOTHING to do with it. I don't look to the mysterious sovereign decrees of God for the answer to this question. That does more to erode my belief in his goodness than anything else. He seems good, but secretly He isn't!? If we've seen Jesus, we've seen the Father. Jesus never secretly gave people diseases - He healed them!
Greg Boyd wrote a classic little book entitled "Is God to Blame?" - it is arguably the best book on the problem of evil that I've ever read. Calvin and Augustine would hate it. Sproul and Piper hate it. But those who want to somehow reconcile that an all-powerful God is also perfect love in a world full of sin, sickness and evil cannot find a better answer to the 'problem of evil' than this short work.