I haven't found any interviews with Tarantino or the rest of the crew to definitively answer this, but I think it's largely about the symbolism involved.
Just before Django killed John Brittle, he is recited scripture from the Old Testament (about justice) to a slave girl, while he whips her. He believes he is right. He believes he is superior to the girl, from a superior race. He treats her a lesser being and nothing more than that.
Django then challenges him and shoots him. His blood pours out of his wound which just so happens to be right in the middle of one of the bible pages. Considering the fact he was lecturing the girl on justice before his death, to me this suggests Tarantino was implying this was the real "justice" being delivered, with his blood appeasing the words of the Old Testament.
So in summary I think the Bible pages are there to demonstrate his belief that he is one of God's servants. I think they also serve a double purpose as his death and bleeding out on top of the scriptures symbolise "justice" being done given his false beliefs.