I'll make this quick:
An argument that gets a lot of play against atheism is the one about Hitler, Stalin, Pol Pot and Mao all being atheists. I hate hearing this because it shows a complete lack of understand for what atheism is -- more accurately, what atheism isn't.
There can be no question that the four of these men commited a vast number of murders. A number of murders so large, it's embarrassing for all humanity that we let it happen. The difference though, between these dictators and your average theistic suicide-bomber or Christian with a U-Haul, is the question of whose hands are bloodied. Who does the dirty work?
You see, Hitler can claim responsibility for the death of 6 million Jews in the 20th century, but he didn't do a bit of work. (short aside: I wonder if Hitler ever killed one person with his own hands.) Instead of Hitler going out with a gun and 6 million bullets, he convinced an entire country to do his bidding. That sentence should throw up red flags in the logic section of your brain.
How can an entire country full of rational people commit such atrocities? Didn't anyone stop to say, "Hey, maybe this isn't right."? Did anyone question the values of the society they were living in? Were there people who doubted the judgment of their leader? The answer is no, and that's why I'm an atheist.
Religion, above all, teaches you that faith is good, the blinder the better. The more subserviant you are to god, the more you give up of yourself, the better of a christian/jew/muslim/buddhist/whatever you are. In religion, you are to taught to bend your knees, not extend your brain. Doubt is something that cannot be afforded in religion, and one thing that is consistantly perpetuated is the belief that relgion is somehow "immune" to doubt and scrutiny, especially of the scientific variety. Religious leaders teach that science should not touch their beliefs, because they know that their beliefs could not withstand the test that science gives.
So think back to Hitler again. The power of murder was within the hands of the people, the whole time. Had there been a movement of individuals who said they would not kill for a pointless reason, that their leader should be questioned, and people should be deciding for themselves what's right and what's wrong, there is a very good chance the Holocaust would have never happened.
Skepticism, a morality independent from political leaders, increduality, logical thinking, human empathy (Humanism) -- the lack of these things are the true reason behind the biggest atrocities of the 20th century, not a disbelief in god. The theists love to use these four men as proof that atheism is bad, but in actuality, those four men are the reason I am an atheist and proud of it.