My Transition from Christianity to atheism began with the discrepancies between the texts and modern science. We know some of these tales are incorrect (e.g. Creation and the Noah myth) and while I acknowledge that many Christians no longer hold these myths up as the truth, there are still many who do. Evolution is not just some hypothesis without foundation and physics concomitant with celestial observation have revealed many truths about cosmology. Early in my life, this realisation led me down a path of learning where I continued to deviate from the religious world view.
With my studies in Ancient History and the Classics, a whole new world opened up regarding belief systems in a former time without our vast wealth of accumulated knowledge. Religion was born out of several factors, but primarily I feel it began as a method of explaining the inexplicable at the time. Individuals then used this in order to create a priestly class, which often became a civilisation's aristocracy, and it transformed into a method of government and control. Religions borrowed from each other and one can trace many literary devices and cultural influences throughout the texts. Indeed, religion demonstrates a pattern of evolution within itself, starting with fertility cults and anthropomorphic deities, later becoming pantheistic and ultimately, monotheistic. Christianity borrows from traditions that preceded it including Akhenaten's monotheism (note the similarities between the Hymn to Aten and the Bible's Psalm 104), and the Epic of Gilgamesh for example, but one can also detect the influence of Hellenistic and Roman concepts throughout the texts. Furthermore, in Roman times, the concept of individual's ascending into heaven was popularised among the early Caesars, and is it merely a coincidence that a roaming holy man in Judea ascends into heaven? The virgin birth has precedents and parallels in earlier Assyrian, Zoroastrian and Egyptian mythology, and had become a literary device similar to finding children floating downstream in baskets.
Many have tried to argue that religion is required in order to maintain social order and personal morality, but this belief too is specious. For I feel that fear of retribution from a god or gods is a poor foundation for one to adhere to a socially acceptable code of conduct. Should it not be borne of a genuine wish to do the right thing? Is that not conceptually more highly developed than the primitive concept of punishment?
I also found the dualistic nature of religious dogma to be rather simplistic and unrealistic. That is, the world is comprised of good vs. evil, however, we know that true evil is rare, and although many despicable acts have been perpetrated by individuals throughout history, many believed they were doing the right thing, and as the example in the OP demonstrates, religious fervour has been employed to justify the most heinous of acts - acts that openly contradict many of a dogma's basic tenets. Religious dogma is often based upon this overly simplistic world view of good vs. evil and most tales are representative of this perceived struggle.
We, as a species have trouble putting away these childish concepts and we cling to them out of fear: fear of our own mortality and insignificance within the universe. We pray to a god and hope that this god will listen to us and do our bidding. We console ourselves when loved ones pass with a belief that they still exist, or will exist again despite the physical world demonstrating otherwise. We place faith in that which we cannot see, touch or hear, while we treat each other appallingly - often using this belief as a justification.
So, in summary, I feel that advances in science, the specious nature of the texts and the simplistic sources of morality have all contributed to my ever growing scepticism, as well as the irrational nature of the belief systems themselves. The more I learned, the more atheistic I became.
It is impossible to prove the existence of a god or gods, and I am astounded that we still cling to these ancient and primitive superstitions and belief systems knowing what we know in this day and age.
I am an atheist simply because there is no valid reason for me to believe in the existence of a god or gods. I cannot prove there are no god or gods, so I cannot state that they don't exist, however, I do view the belief as being a fantasy based upon a dearth of evidence, and I reject it as such.