I'm sure this is not terribly original, but here's my story.
One day, spaceships appear in the sky. Appearing on every television screen, radio, and pool of water, the ships broadcast the following message to everyone:
"Greetings, citizens of Earth! We are a race of life forms so vastly superior to you that our ways cannot be understood by your puny human brains. We also possess knowledge of morality that is advanced far beyond your own understanding and cannot be refuted by any of your Earth philosophers.
"According to our high moral standards, which we cannot explain to you, you all deserve to die the most painful deaths imaginable. We shall now execute this sentence. Your insides will be melted, and your eyes will explode in their sockets. Your children and spousal units will be vaporized before your eyes. Your planet will then be incinerated.
"However, our laws also require mercy, and therefore you will have one chance to save your own miserable lives. If you become our slaves and do as we say from now on, you will be transported to another planet and allowed to survive. However, your unrepentant family members will still remain behind and be destroyed."
I don't know about you, but I've seen this movie many times. It involves the entire human race justifiably uniting to fight off the alien menace. Depending on what kind of movie it is, either we're victorious or everybody dies for nothing.
Now the question I have is: How is this scenario any less outrageous if you rename the alien beings "God"?