An argument frequently made by atheists against theism is that believers in God are often guilty of horrific evils. This sort of argument is well represented by the following quote from Nobel laureate Steven Weinberg, a physicist:
Frederick Douglass told in his Narrative how his condition as a slave became worse when his master underwent a religious conversion that allowed him to justify slavery as the punishment of the children of Ham. Mark Twain described his mother as a genuinely good person, whose soft heart pitied even Satan, but who had no doubt about the legitimacy of slavery, because in years of living in antebellum Missouri she had never heard any sermon opposing slavery, but only countless sermons preaching that slavery was God's will. With or without religion, good people can behave well and bad people can do evil; but for good people to do evil—that takes religion. (quoted from "A Designer Universe?"as cited in Wikipedia).
In addition to the examples cited by Weinberg, atheists may refer to such events as the Inquisition, the Crusades, and the 9/11 terrorist attacks on the U.S.
Weinberg's words seem quite damning; however, I would argue that careful analysis would demonstrate that his argument is not as strong as he seems to think it is.
First of all, there is the matter of the language Weinberg uses--"good" and "evil." Since Weinberg is an atheist, he presumably rejects the idea of moral absolutes. After all, if there is no God, there is no moral lawgiver who establishes the ultimate standards for good and evil. Moreover, as an atheist, Weinberg is presumably a materialist, one who believes that only matter exists--thus, non-material things such as moral laws cannot exist. Furthermore, atheism generally presupposes naturalism, the notion that Nature is all that exists. Nature does not provide any basis for morality. Consequently, for an atheist like Weinberg, it can only be the case that good and evil do not really exist. At most, such words simply indicate the speaker's approval or disapproval of some action or state of affairs--they cannot refer to any absolute standard. If that is true, then Weinberg has no right to be talking about "good" and "evil" in the first place.
Second, Weinberg is demonstrably wrong in asserting that "for good people to do evil--that takes religion." Actually, ostensibly "good" people have done evil in the name of all sorts of things, some of which I suspect Weinberg would approve of--science, reason, and even atheism. For example, the infamous Tuskegee syphilis experiments on poor African American men (who were never treated for the disease) were justified as a form of medical research.The so-called Cult of Reason in Revolutionary France led to the ransacking and desecration of churches. As for atheism, it forms one of the fundamental tenets of Communism, and it was under Communist governments in countries like the Soviet Union, China, and Cambodia that millions and millions were the victims of what can only be described as political murder. In short, it doesn't require religion for "good people to do evil"--it can take almost any sort of rationale, no matter how admirable, to justify evil.
Third, an argument can be made that that no one is truly "good"--that there are no genuinely "good people." In fact, this is what one religion, Christianity, teaches quite explicitly. In the Christian scriptures, the prophet Jeremiah tells us that "the heart is deceitful above all things and beyond cure" (Jeremiah 17:9) and the Apostle Paul, quoting the Book of Ecclesiastes, insists that "there is no one who does good, not even one" (Romans 3:12). From the Christian perspective, even "good people" are capable of evil--because they are not truly "good." Thus, it should not be surprising (though it is disappointing) that even "religious" people, who profess the highest moral standards, can believe or do quite horrible things. This is the reason we all are in need of the grace of God manifested in Christ--without it, we can never be truly "good."
Fourth, the fact the some people use religious arguments to justify evil doesn't necessarily make those arguments valid even within the context of that religion. Take the example of southern Christians justifying slavery that Weinberg alludes to. It is true that the Bible does not explicitly attack slavery--indeed, the Old Testament sanctioned a type of slavery and the New Testament urges slaves to obey their masters rather than rebel against them. However, Old Testament slavery was seen as being temporary in nature--a means for the poor to pay off their debts--and was not racially based, unlike slavery in the American South. And although the New Testament does not require the liberation of slaves, its emphasis on the equality of all believers (including slaves) ultimately led to the conviction among many Christians that slavery was incompatible with their faith (also, it should be pointed out that slavery in New Testament times, like in Old Testament times, was not based on race). Thus, Christianity did not really provide a rationale for slavery in the American South, and the so-called biblical arguments for Southern slavery were never really valid.
Finally, there is the "rest of the story" that Weinberg doesn't tell us about. While it is true that Mark Twain did reject Christianity, at least in part due to what he saw as the hypocrisy of Christians, Frederick Douglass ended up claiming Christianity as his own. Apparently, Douglass came to the conclusion, as did many white abolitionists of the time, that genuine Christianity, what he called the "Christianity of Christ" (as opposed to the "Christianity of America"), was antithetical to the holding of men and women in bondage. In short, despite suffering at the hands of "good people" who used Christianity to justify evil, he realized that the problem was not Christianity, but rather the evil nature of human beings who failed to conform to the genuine teachings of that faith.
To conclude, Weinberg's argument--common among atheists-- that belief in God should be rejected because it leads to "good people" doing evil things is not particularly convincing if carefully analyzed. However, the strength of its emotional appeal cannot be denied. Thus, it is not surprising that this sort of argument frequently appears when atheists try to undermine belief in God.
Image of Steven Weinberg from commons.wikimedia.org