A fair question, but it wasn't a single part of the Bible; it was the overall thrust and tone. I finally realized what the Bible really was by considering its known history. Remember, you asked the question.
The Bible does not tell us of God. It is a set of written transcriptions of stories that ancient people told about their beliefs in God using an allegorical story-telling style. The Bible is NOT eyewitness testimony. It is a set of stories meant to illustrate a point.
That bears simple repetition: The Bible is NOT about God. It is about the beliefs held by simple, primitive, superstitious people; beliefs in a being they called God.
They believed in God because they didn't have adequate scientific knowledge to explain their world. So as happened with ALL similarly primitive people, they made up stories to explain their beliefs to their children.
In that purpose, it has the same exact status as Aesop's Fables or Grimm's Faerie Tales. The Bible lists stories and oral traditions meant to amaze, amuse, and admonish small children who didn't have radio, TV, newspapers, smart phones, or video games. These were the bedtime stories of a 2000-years-ago culture. The storytellers were the lore-keepers of the primitive shepherd tribes and a lot of the stories in the Bible are just written copies of their stories preserved in ink on papyrus, parchment, or vellum.
It is important when considering this to note that a LOT of the stories of the Bible parallel stories from earlier cultures. The story of Jesus parallels the story of Horus (of Egypt) - which in turn parallels the story of Gilgamesh of ancient Babylon. Satan as the ruler of Hell parallels stories of Hades, the ancient Greek god of the underworld. The name Hell, though, comes from Hela, the NORSE Goddess of Death. We talk about "Hell freezing over" but in Norse mythology, it already IS frozen and miserably cold because it is in an obscure corner of Frostheim. And the name Hell is a shortened form of the name Hela, who reigned as queen of that region. The flames of Hell are derived from the pits of Tartarus, the Greek version of the bad place in the afterlife. The great flood is a crib from an ancient Sumerian story of flooding, and even THAT story isn't original.
Adam, your problem is that you don't read enough books to see beyond the confines of the Christian culture. You can't see the parallels because you have avoided the books that reveal them. You will probably claim lack of interest. But you did ask why reading the Bible turned me towards atheism. The simple answer is that I have read enough to see the parallels and draw my own conclusions rather than have some preacher tell me what to think.