Why do most conservatives claim that morality without God is dangerous? Short answer: because Man is prone to error, imperfect, and capable of corruption. To answer at length, let's look at the broad field of morality and moral codes.
All moral codes are based on some standard, answering the question: if an action is good, bad, or merely neutral, then why is it so? Good, by what standard? Bad, by what standard? Neutral, by what standard? There are only two, possibly three answers: something can be good, because someone other than man (God) declared it to be so. Or, something can be good because Man declared it to be so. The philosopher Ayn Rand proposed a third definition, or so she claimed: what is good or evil is dependent upon how an action affects human beings. What harms an individual is evil. What benefits him, is good. Ayn Rand's ideological dependence on the reliability of human rationality aside, in other words, what is good or bad is determined from the declarations of God, or from the feelings, laws, customs, etc., of humans. In other words, what actions are good or bad is determined either objectively, or subjectively, from the perspective of humans, who act.
To claim an action is good merely because I said so, based solely on my feelings, is to claim that my words (and the feelings underneath those words) are sufficient to define what is good. If I were God, then this would be justified, but since neither I nor any individual is anything but human, such a claim is preposterous. No man created the world. No man is God. Even so, if all I do is simply live my life and act according to what I feel is good, that is one thing, perhaps a good thing, perhaps not. On the other hand, if I-or any other human- require by force others to act solely according to what I feel is good, that is to arrogate my identity as God. In other words, when morality comes from Man, it is subject to human imperfection and corruption. When morality comes from God, then and only then does it have any chance of being true, correct, and perfect. Notice the assumptions here: that humans normally wish to lead happy, flourishing lives, and the purpose of ethics is to answer the question how to live such a life. The answer is what we call a moral code, or in general, morality.
Conservatives in general tend to think that human beings can be perfected only in a relation to God; liberals tend to think that we can be perfected by evolving into perfection. When you start talking about the specifics, however, the sparks fly. When you postulate the imperfections of Man, basing a moral code on Man's conscience, laws, culture, on anything other than God, you are inviting the Holocaust. With Man, there is the possibility of injustice. With God, there is the inevitability of justice. So the atheists can have their faith in human rationality and reason: I will have my faith in God. Just as America is a nation of laws and not men, morality is based on God, not men.
