I love telling this story to the religious, watching them squirm and say it is not in the bible, then giving chapter and verse.
You're right they almost always say "out of context" until you ask in what possible context is mass child murder acceptable? Then follow that up with, oh yes in Egypt when god killed all the first born.
This usually results in them shouting "Fuck off, you'll burn in hell!" the ultimate argument of the christidiot.
No other book can make atheists faster than the Bible when read without coaching from a religious instructor. When my daughter was about 12 she started asking questions about the Bible. I handed her one and told her to start reading at Genesis 1, a chapter a day, all by herself. She got to where Lot had sex with his own daughters and that was it for her. I don't think she ever picked up another Bible ever again.