Probably the single biggest passage that did it for me was Numbers 31. This is the attack on the Midianites. The Israelites believed that two Midianite women had “tempted” two men to worship their god instead of Jehovah. As a result, Moses commanded — supposedly because God told him to do so — that all Midianites be slaughtered. Except for the virgin females, who were divided up among the soldiers as the spoils of war. And it occurred to me that this is something that even Adolf Hitler and Joseph Stalin didn’t do. Even Nazi Germany didn’t force the women they’d conquered to marry the men who had slaughtered their fathers and brothers. That’s how barbaric it was, it went beyond anything even the worst human beings would do. And this is the same being that supposedly commands us to love one another and to do unto others as we would have them do unto us? It just didn’t add up for me.To make things worse, a lot of fundamentalists believe Moses is being rewarded with eternal bliss for leading a "holy" life.
As for the evils of Adolph Hitler and Joseph Stalin, the fact that they didn't order anything like this suggests that even our archetypes for complete monsters are still less evil than the god of the Old Testament. Even if you try to detach the god from the barbarism, it still makes a far more horrifying vision for humanity than anything modern fundies can confabulate for the "degradation" of our civilization. How exactly are rock and roll, Harry Potter, and equal marriage rights going to produce anything even remotely as bad?