One of the most formidable arguments against the existence of God is the extent of evil in the world - either God's not very good, or he's not very clever. Or perhaps he's stranger than we sometimes imagine.
One of the strongest responses to this argument is to ask what is the definition of "good" and "evil" on which the argument depends, and where did it come from? We normally think of morality as something which defines things we "should" or "should not" do, despite how we might wish to do. Without God, ethics seems to be nothing more than how natural selection has led us to think, and there is no "should".
Christopher Hitchens has written with some passion about the evils of religion, and in a 2007 debate with a christian theologian, his views were put to the test (a downloadable pdf file is also available).
Read it for yourself and make your own judgments, but I thought Hitchens came across as evasive and unable to provide any objective basis for his ethics.