Once I gave up (or couldn't rationalize) a belief in Jehovah, I gave up believing in any sort of God. So, perhaps at heart, I always was an atheist, I just didn't realize it.
As a child, I used to wonder: If God is so wonderful, why does he make it so hard to serve him? He is, among other things, a terrible communicator. And he must really like to watch people suffer. There's no way that the "preaching work" can ever reach everyone on the earth, there are so many people born every year, many in places where the JWs (or any other church) can't preach, that there would never be an end to it all. That made me think that if God does exist, he's not really a very nice person. From that point of view, it's an easy transition to full on atheism.
It was a great relief to let all that god-belief go!
All that doesn't mean I don't believe in being ethical and living life to a higher standard. It just means I'm not looking to an outside "god related" source to tell me what to do.