I became an atheist before I decided I was no longer a JW, although by then I had long since faded. I just kept telling myself that I'd return at some point, not realizing that somewhere in my head, I'd given up on the organization. The bright side is that I did not seek out "apostate" material until after I had left, so there was no question about whether or not I'd been 'tricked' into leaving. It took a good long time, but I figured it out on my own. Basically, I tried to make sense of god as both a being and a concept, and found that -once I dropped the presupposition that he was real- I couldn't.
I don't fuss with whether I am agnostic or atheist. I am open to the idea that a god or gods exist, but so far I have not found one that makes sense to me. The ones that are supposed to have a deep interest in us, yet never show their faces (except, apparently, for the occasional piece of burnt toast), who desperately need us to care about them (on pain of eternal punishment), who blame us for their bad designs... they don't make sense to me. A god that is indifferent to us, who sees us as a science experiment and is not at all concerned with our individual well-being... that makes more sense of the universe and world we live in. But in that case, I'm not obligated to worship it, believe in it, or care about it.
I don't know if the universe requires a creator, or if it's some kind of accidental self-regenerating thing. I leave that to the astronomists and physicists. Without the concern of an afterlife or the belief in any of the gods we know, I don't feel a need to worry about it. It'd be nice to know, but life is short and it's pretty awesome when you're enjoying what is instead of worrying about what might be.