Debunking God

Darwin's discourse on religion seemed ambitious to me, part related as opinion and part related as fact, which can be a dangerous mix. Perhaps he wasn't concerned about the reaction his discoveries would cause, but it seemed like he was asking for trouble when he said "the old argument of design in nature, as given by Paley, which formerly seemed to me so conclusive, fails, now that the law of natural selection has been discovered" (431). Where the old argument allowed for the coexistence of science and God, it seems that Darwin himself is taking credit, as one of the couple discoverers of natural selection,  for debunking the God myth. 

Darwin's disbelief in the Old Testament seems based on his own reasoning, characteristic because much of the Bible has little supporting evidence that Darwin could observe and understand. Could it be possible, then, that if Darwin had been looking for a different outcome, that he would have found alternative reasoning for his findings? Where does the clockmaker theory fit into everything at this point? Darwin considers "happiness" as a potential proof of a benevolent deity, but isn't the idea of happiness itself a societal construction?