It would be helpful if you explained in more detail what you believe to be the inherently destructive nature of belief in a god and the tremendous evil this belief results in. Are you limiting this destructive evil to the typical examples of the use of religion as a vehicle for individuals to obtain power (The Crusades, The Inquisition, The 30 Years' War, The predations of the Thuggee Sect, Witch burning, Islamic Jihad, etc), or do you include some evil destruction of the self?
If the former (or if you simply include it in the whole evil package) do you contend that war, torture, banditry, conquest and murder would not exist absent a belief in God, or at least been significantly reduced throughout history?
Am I correct that your argument concerning heaven and hell is that being motivated to do good deeds and avoid bad ones by a promised fate in a life after death is not necessary, and that to the extent that it has been, it's cheapened morality?
If so, how to you know the extent to which these beliefs have actually motivated human beings over history, and that they are in fact not necessary?
To Olivier's point there have been Godless societies in the world (or at least as close to ones as we may be capable of) and they hardly displayed a a core natural morality that was appreciably elevated above the religion based morality of societies where belief was either a cornerstone of their development or at least pervasive in practice. You seem to be contending that for quite a long stretch of our history, humanity didn't have the knowledge to understand what rules of behavior are beneficial to society and therefore had to rely on priests or sacred texts to inform it. That's an interesting proposition that I would expect you to be able to support.
We may now know more then our ancestors of not too long ago about what rules are good for a well functioning society, but what evidence is there that this knowledge alone (with, of course associated laws and law enforcement) is capable of effectively regulating our behavior so as to produce such societies? Are these rules the equivalent of morality?
Finally, what do you acknowledge as the good that has come from belief in god or religion? I don't know that we can judge the validity of your contention that the evil far outweighs it without having a better idea of what you believe both to be.