Take a look at this picture. It's been said that Christians doing good only do it because it's an order from a superior, whereas atheists do it simply because they love and choose to be moral, using free will. Sounds nice, huh?
But stop for a second. What is free will? We know that all actions are responses to a stimuli. We know that we are vastly complex biologic machines. You see an orange. Light has reflected off of it and hit your eye. Nervous impulses travel along your neural pathways and interact with cells in the brain. Chemicals fire and change in response to this stimulus. You eat the orange.
Your mind is a vast array of cells and chemicals, all responding to physical forces. But what is free will? Willpower supposes that some disembodied "force" can change the chemical reactions going on inside our bodies. Instead of eating the orange, you can "choose" not to. But free "will" is the equivalent of the spirit—a nonsensical illusion to any true atheist. Can you prove that you could have done anything different? No. Can you pinpoint this "force" that lets you "choose" to behave this way or that? No. Can "you" somehow "force" the chemical reactions happening in response to stimulus to react differently? To have one chemical outcome instead of another? No.
Does an atheist, then, really "choose" to be good or bad, in a universe where we are simple science? No. There is no good. There is no evil. There are the illusions of them—intangible thoughts; nothing real. To believe that one has some "will" outside and superior to biophysical mechanics is truly superstitious—and yet inevitable and pointless. The logical conclusion for the atheist is that everything is casually determined. Two famous atheists really summed it up, saying,
"“You, your joys and your sorrows, your memories and ambitions, your sense of personal identity and free will, are in fact no more than the behaviour of a vast assembly of nerve cells and their associated molecules.” —Francis Crick
“Given the state of the universe at one time, a complete set of laws fully determines both the future and the past. That would exclude the possibility of miracles or an active role for God....It is hard to see how free will can operate if our behavior is determined by physical law, so it seems we are no more than biological machines and that free will is just an illusion.” —Stephen Hawking