First off, I'll explain my theory of the majority of atheist views. Starting out with a fact, it is impossible to prove the nonexistence of something that does not stand within well defined boundaries, such as god. While there are many different types of atheists, I think that one thing is generally believed among atheists; if God interacts with our universe in any way, the effects of his interaction must have some physical manifestation. Therefore, his interaction with our universe must be, in principle, detectable. I'm going to assume that a natural response to this statement is that god is essentially undetectable, by that means, god would have no physical manifestation on this plane of existence. I'm sure that atheists would argue that if god does not interact with our universe at all, it is of no importance whether he exists or not. In terms of existence; A thing which cannot even be detected, in principle, does not logically exist. Relating this to the agnostic belief; there is simply a lack of proof, therefore, the lack of belief. Understandable in most cases.
The first conclusion that I have derived over the years is that reality is not decided by logic. Even if you could rigorously prove that God exists, it wouldn't actually get you very far. It could be that your logical rules do not always preserve truth, that your system of logic is flawed. It could be that your premises are wrong. It could even be that reality is not logically consistent. Unfortunately, the only way to find out what is really going on, in a sense, is to observe it. Logic serves as a means of observing - helping you find a way to "look". A few years ago, I ran into this example, which has proven quite helpful:
"Person A begins by making a contentious statement. When person B points out that it can't be true, person A gradually re-defines the words he used in the statement until he arrives at something person B is prepared to accept. He then records the statement, along with the fact that person B has agreed to it, and continues. Eventually A uses the statement as an "agreed fact," but uses his original definitions of all the words in it rather than the obscure redefinitions originally needed to get B to agree to it. Rather than be seen to be apparently inconsistent, B will tend to play along."
I remember that the author of this referred to it as the redefinition game, where most philosophical discussions end up. This is part of the problem when discussing the situation based on the many definitions of god that people can have. Then, due to that situation, you have to define morality, and its relation to belief as a whole.