The subject here is not to debate the existence of God; rather, it is the belief that leaving the JWs will lead to atheism while abandoning ones good principles.
In reality, for many, it is actually the opposite: They first stopped believing in God (or a personal God), and then left the witnesses. In addition, for many, leaving the JWs have opened the door to even higher principles. For instance, many now take time at their disposal to participate in various forms of charity. Some have rekindled with long lost relatives and provided loving assistance in circumstances where the disfellowshipping arrangement would have prevented them to do so.
Many religion scare their followers in telling them they that they are bad people and the only reason they don’t submit to their true desires is that they are reminded on a regular basis, at church, no to succumb to sin. However, this belief obviously fall apart when someone resists to persecutions, in isolations, for many years.