When I left Christianity, I immediately thought of myself as an atheist. Is it "an atheist" or "a atheist"...I've seen it both ways?
After time, I started to call myself an agnostic atheist and generally leaned towards just calling myself an agnostic, because, well, I can't prove one way or another, if there is or is not a god. Part of the reason I think I tended to say I'm an agnostic, was because it was easier on the ears of those who were my belieivng theist friends.
It's taken me four years to come back to realizing that when push comes to shove, I'm an agnostic atheist but the short answer is, I'm an atheist because I don't believe in or worship any god(s).
I think for those who were once Christians, the transition to finding a new label (because it seems the world must have them) can be a very difficult one, scary even.