Is kindness right because God commands it or does God command it because it's right? Thus runs the ancient dilemma posed by Socrates to Euthyphro some 300 years B.C. Philosophers have come down on both sides of the question with most atheistic philosophers arguing that if God commands it because it's right then we don't need God's command and thus we don't need God. Theistic philosophers argue that an act is right not because it's commanded by just any deity but because it's commanded by an omniscient, morally perfect Deity.
I think the theist has the better reply. If the moral authority is ab defino all-knowing and perfectly good then anything It commands would be necessarily good and It wouldn't command anything that wasn't good.
God commands certain behavior for the same reason that a car manufacturer puts a maintenance manual in the new car. Just as failure to operate the car properly ruins the car, certain behaviors - like cruelty - are harmful to human beings and certain other behaviors - like kindness - enhance human existence.
God commands us to enhance human existence because he loves us. To mistreat others is wrong not because it's wrong in itself, but because God obligates us to refrain from hurting those He loves. It's wrong, and this is crucial, only because it violates the will of God. If there is no God, as the atheist maintains, then whether the atheist decides to be cruel or kind is of no moral moment. There's nothing which obligates us not to hurt others except our own subjective preferences.
Just as the sun generates light and heat which would not exist if the sun ceased to exist, so, too, goodness is ontologically dependent upon God. It's an attribute of His essence. If God did not exist neither would goodness and neither would any obligation to perform one act rather than another. We are obligated to do justice and kindness only because God demands it of humans. Take away God and human beings would be animals just like sharks and wolves for whom moral categories and moral obligation simply don't exist.
So, its not that atheists can't be kind. They certainly can. It's that, on atheism, kindness is neither right nor wrong. It's no different, morally speaking, than cruelty. It's simply a personal preference like one's preference for Pepsi rather than Coke. In a Godless world morality is just a matter of taste.RLC