Atheists can’t prove there is no god
Of course not; negatives are not proven. But the existence of any kind of creator deity is ruled out more and more, made less and less probable, as evidence accrues for natural processes such as evolution, plate tectonics and the big bang. These scientific theories do not disprove the existence of a deity; they are an explanation that does not require one. Yawn-inducing stoner conundrums like "Well, what came before the big bang?" and "If man came from monkeys why are there still monkeys?" are not the atheist's problem; one can easily consult a science book for these answers. Whether one still believes in a god after that—Well, that's the real trick, isn’t it?
Again, this has no bearing on the existence of a god. But I find it more than a little ironic that theists who believe the entire universe and everything in it was created solely for the outcome of their own existence to then accuse an atheist of being arrogant. Now, this can be true; nobody likes a smartmouth, as I’m sure your mother told you. Sometimes atheists or even believers of another faith can unfairly malign some theists or offer glib criticisms of faith; atheists hold no monopoly on good manners. But anyone who’s actually seen/heard Carl Sagan or Richard Dawkins—two critics of faith who are often branded as “arrogant”—take religious beliefs to task knows they are the very picture of civility and polite debate. It just seems that theists just don’t like a flat-out rejection of their beliefs delivered in calm, reasoned, and rational terms.
Atheism leads to nihilism
Again, if atheism leads to nihilism, so be it; atheism is a position about reality, not a prescription on how to live. Personally, I think theists are the nihilists: for them, without a deity, nothing means anything. They cannot imagine how to make sense of a cosmos without some sort of supernatural agent at work. Beyond their god is the great yawning abyss. Take away that god, and they shudder to think... In defiance of that oblivion we should find our common humanity,whether theist or atheist. But if theists can’t figure out that genocide or rape or lying are wrong on their own and need a trans-temporal everywhere-and-nowhere eternal invisible entity (and yet strangely confined to a Middle Eastern locale roughly the size of, what, New Jersey?) to tell them so, well, that says more about their moral compass than the atheist’s.