Hey!
Alright, this is going to be an EXTREMELY controversial answer, but, hey, this is a controversial question. And please don't hate me or insult me because my beliefs differ from yours. Please?
First of all, I'd like to establish the idea of free will. Free will is the capability to choose who you want to be, what you want to do with your life, and how you want to make those things happen. You cannot, I repeat, CANNOT have free will unless their is a choice. Otherwise, if there is only one choice for you to make, you are not really choosing it. You are no better than a machine, an automaton, programmed and forced to do one thing, and one thing only, never choosing anything for yourself. We'd all look the same, have only one color, have the exact same personality, and the exact same way of expressing ourselves. Just like robots do.
Free will sounds better, when put that way, don't you think?
So, in the beginning, Lucifer, Adam, and Eve all had a choice. They could follow the perfect path God had set before them, or they could deviate from that path and make their own.
Now that we have that covered, let's start at the beginning, shall we? Genesis 3:1-4 read:
"Now the serpent proved to be the most cautious of all the wild beast of the field that YHWH had made. So it began to say to the woman: "Is it really so that God said YOU must not eat from every tree of the garden?" At this the woman said to the serpent: "Of the fruit of the trees of the garden we may eat. But as for [eating] of the fruit of the tree that is in the middle of the garden, God has said, 'YOU must not eat from it, no, YOU must not touch it that YOU do no die.'" At this the serpent said to the woman: "YOU positively will not die.. For God knows that in the very day of YOUR eating from it YOUR eyes are bound to be opened and YOU are bound to be like God, KNOWING good and bad.""
What, exactly, was Satan saying to Eve, through the serpent? Did he not say that she would become 'like God' if she ate from the fruit of the tree of knowledge of good and bad? And, if we are 'like God', do we need God himself to lead us through our lives?
THAT is what Satan was perpetuating to Eve. He was, in effect, telling her that humanity does not need God to make their decisions for them. That humans themselves can decide what is right and what is wrong and not have any need for God to lead them to the right way.
Well, that was an absolute lie (no surprise there), as proven throughout mankind's history. But, Adam and Eve did eat from the tree of knowledge of good and bad, saying that they, too, did believe that they did not need God to guide their steps.
So, as a just and fair God, what do you think he should have done in that situation? Do you think he should have destroyed Satan, Adam, and Eve right then and there and started all over again from scratch? Wouldn't that have solved the problem and not caused so many so much suffering?
But what about the angels in heaven? They were looking on as God dealt with these usurpers. If God just started from scratch, wouldn't the questions just have stayed in their minds, making them constantly question and wonder, "DO we need God?" And what about those future creations God might have made? What if they someday questioned God's right to rule?
Clearly, this question of God's right to guide his own creation needed to be completely settled. Besides, giving mankind, and even Satan the Devil, a chance to prove that their claims were true, that they didn't, in fact, need God, was only a fair and just thing to do.
And I believe in a fair and just God.
BUT, God set a time limit on Satan and mankind to prove that they didn't need God. That time limit started when Eve was created (after that God started his day of rest, no longer creating anything) and continues until God's 'day' of rest ends. Remember, time is relative. You can't conclude that the way we measure time is the same as God measures it. After all, our measurement of time is based off the sun and the moon, and these are only creations of God. So, we have no concept as to God's view of time. And, also, remember that in the Bible that when it comes to Eve's day of creation, there is no way of tracing a date and time. So, we have no real idea of when the 'end' will come.
So, as I'm sure you know, Jesus Christ died to balance out the scales of Adam's sin in the beginning. That blot on our heads is now gone. BUT, we still are in the time set for us to prove God's right to rule his creation (or the lack there of, but that's never going to happen). THAT is why there is still sin. We still have something to prove. Until the question is settled, we have to tolerate imperfection. After all, it's a path we set ourselves (or, at least, that Adam and Eve set for us). And cancer, diseases, starvation, and all the other horrors of the world are just side-effects of imperfection. In fact, that's EXACTLY what cancer is. An imperfection of the cell.
Just think of how far we are in time since Adam and Eve made the choice that cut all of us off from God. Now, think of a laptop computer. If you cut the cord, the power quits coming in. The laptop runs on the juice it has stored up inside itself for a little while, but gets slower, the lights go dimmer, and it makes more mistakes. Finally, it goes dead. People are like that, since Adam and Eve cut off the connection to God.
Anyway, that's my thinking on the matter. It might not be completely factual. You should look inside a Bible, and draw your own conclusions. Also, if you would like to hear more on this subject, might I suggest the article entitled Jesus Saves--How? that can be found at the following link:
http://www.watchtower.org/e/20011115/article_02.htm
I hope that makes sense to you, and that it helps settle things in your mind.
Luv ya,
Tashi :)
P.S. Sorry, I know this is really, really long, but I noticed no one explained how disease could be a side-effect of imperfection or free will (which explains AIDS, famine, etc.). Remember my computer analogy? Well, think of a perfect immune system. It could fight ANY disease, right? But we're stuck in imperfection because we were cut off from God. No perfect immune system for us.
Hope all that makes sense!
P.P.S. I'm sorry again. I have to correct a statement you've made. God did NOT create Satan. God created Lucifer. Lucifer CHOSE to become Satan (which means 'resist'; appropriate, because he is the ultimate resister of God's rule), and God did not create evil. But there is still the choice, again. You have to have a choice for free will to exist. You can choose good or evil. Because if there was only a good to choose from, it wouldn't be a choice. Hence, no free will.
Anyway, just had to add that in, too! Hope all that helps you! Have a nice day!