I know that this isn't exactly an answer to your question since I'm not an atheist, but here goes.....
I'm a Deist. For years I was an agnostic who believed that there was no definitive proof that a god existed, but there was no proof that he didn't exist either.
The reason I wound up a Deist is because my mind needed an answer for existence. How could we be here if something didn't start the ball in motion?
For those of you who don't know what Deism is, here's a short answer for you. Most believe that the "supreme being" or "first cause" simply acted as a clockmaker. It created the mechanism for the universe, set the clock, started the pendulum swinging, and went on vacation without his cellphone or his beeper (prayer). Deists don't pray to a god, because he/she/it isn't active in our daily lives. I tend to think that the "clock" sometimes needs maintenance, so it'll enter the picture again as needed to check up on things, but otherwise, the universe is left to its own devices.
Deists see this "god" as a dispassionate being. Therefore, the idea that he/she/it couldn't exist because of the evil things we see and experience in the world is pure nonsense to us. I think the idea of a loving god as put forth by the Xians drives some people to jump past the possibility of a neutral and disinterested first intelligence and go straight to claiming he/she/it doesn't exist.