*I still don't follow the reasoning there. I don't see how the fact that people choose to do evil -- which, clearly, some do -- means that there must be some malevolent entity... coordinating it all?*
People were created in God's image to dominate the earth, evil came in when Adam gave authority to satan when he sinned. Before he turned over his authority, Adam was all-good, innocent.
*Couldn't all the bad things in the world happen because of the individual choices of the people involved to cause harm?*
You're right in a sense, we all have the power to choose to cause harm or bring good. It is our choice now that there is evil in the world.
*What about bad things that happen which are considered "acts of God"?*
I don't pretend to be able to pin-point the reasoning behind God's actions, but I do know this... (and this may not answer your question)
God does bring judgement on the wicked. He has to, to sustain the purity that He embodies. An All-Good God cannot allow evil in His presence, He can't even look at it. Example: the "Act of God" of all time - the flood in the Old Testament. God had to rid the world of evil... he almost wiped out life completely (again?) but he found Noah was good so He spared him and his family.