It seems like atheists are asked that question often. As if they believed in some god enough to actually hate it. First I always ask back, "which god are you talking about?" (It's usually Yahweh/Jesus.) I think that's a good question to ask in response to most questions about "God". Note the capital "G" implies a particular god that the target of the question is assumed to know.
Another common criticism of atheists is that we are always talking about gods! How ironic! I suppose a certain segment of atheists really do have some kind of hate or obsession to the idea they were brought up with. They feel traumatized and feel the need to get some kind of revenge. I was never raised religious so it's hard for me to relate to that. I do enjoy discussing ideas of god, cosmology, and philosophy, but not all my discussions with atheists are about those things. Sometimes we talk about mundane things like TV shows, or the lowest form of conversation: gossiping about other atheists. Very sinful indeed.
One final point I like to make to theists, is that I really do wish some kind of god did exist! I'm certainly not against the idea. I'd love for there to be some kind of cosmic justice to punish the wicked, or some kind of afterlife. Sounds good! I'm not sure I would go for the Christian god though. Yahweh is kind of a nasty character, and despite Jesus's good points he did introduce us to the idea of eternal torture. But my wish for a nice god isn't going to cause me to actually believe in it. My brain doesn't work that way. Heck, it would be great if Santa Claus existed too, or Superman for that matter. Humanity is in dire need of a true god to fix things up. Since we have no evidence of a cosmic helper coming anytime soon, we should get our act together ourselves. So stop killing each other and messing up the ecosystem!