But there are some things that don't make sense to me. I don't understand why God needed Jesus to die for our sins. People say that God's justice demands that the price for sin be paid, but how is it justice for someone who didn't commit the sin to pay the price?
It isn't justice, its mercy and self-sacrificing love.
If I rob a bank and get caught, no-one would think it was ok for my mum to say, "I love my child, don't send her to prison, I'll do the time instead." Why didn't God just write the debt off?
I would expect people would think such a mother very noble, and selfless, and loving. They would appreciate the sacrifice she's willing to make for her child. Justice wouldn't be served, though, if the bank robbery wasn't punished. The guilty man must pay for his crime. Likewise, a holy and just God can't look upon sin, upon evil, and simply write it off with an "Oh, that's okay. Forget about it." Doing so would be distinctly unjust and would make accommodation for sin, which a holy and just God cannot do. Instead, God satisfied both His holy justice and His loving mercy by paying the punishment for our sins Himself.
Also, you know a couple of times in the bible God destroyed cities/the world and killed all the people because they didn't turn out how he wanted, and changed some laws, I don't understand how this can be. If God is omnipotent and omniscient then surely he would have known how things would turn out and would have acted differently to begin with instead of changing things that went wrong later?
That's how
you might have done things as a finite, imperfect creature, but this isn't how an infinite, perfect God chose to do things. He doesn't seem to be particularly eager to explain every little detail of His thinking to us. We couldn't comprehend it even if He tried. It seems He wants us to simply trust Him when He isn't forthcoming with answers to all our questions. Some things He has made perfectly clear; other things, well, He leaves us to wonder about.
What would we be like in a world where there was no evil, no pain, no death? Since all of these things have come out of our capacity to choose between good and evil, between obeying God and not obeying Him, in order for us to live in a world free of pain, death and evil God would have to suspend our ability to choose between good and bad, between loving Him and hating Him. He would have to make us into human robots programmed only to love and obey Him. Does that sound preferable to our present state? It doesn't to me.
If God simply went along behind us and cleaned up all our sinful messes; if He went about undoing, or negating all our sin and its consequences, He would, in effect be assisting us in sinning. But a God who let us sin with impunity wouldn't be holy or just, would He? No, He would, in fact, be evil. Moreover, a God who would do as I've just described would be effectively negating our freedom of choice as well. Neither of these things I would want from my God. Would you?
Also, if God created everything, then evil must have come from God. I know people say it's because Adam and Eve disobeyed God, but angels/Satan had already disobeyed God, and anyway God created them the way they were. Free will doesn't explain it, if there wasn't any evil to begin with then we could have free will without the choice to choose evil.
In a sense, you're right: Evil did come from God. I think, though, that it would be more correct to say that only the
potential for evil came from God. Imagine a man who makes a carving knife. In order for the knife to do what it was intended by its maker to be and do, it must have a very sharp cutting edge. Now, although the knife maker intended the carving knife to carve wood, its cutting potential may be used to do other, harmful things. In the hands of a murderer, such a knife may be used to kill someone. This was never the intent of the maker of the knife, however. His intent was simply to make the knife capable of carving wood. In the same way, in order to make us as He intended us to be, God had to give us a "sharp cutting edge" that has the potential to do good and to do harm. It is never God's intent that we should use our "cutting edge" - our intellect, our self awareness, our free moral agency - to do evil, but the potential for us to do so must necessarily be there in us for us to be as God intends.
Peace.