Notice the difference between the attitudes of the early writings, when Mohammed was young and had only one wife (she was rich and he was dependent on her). He was the peaceful Mohammed, the "Mecca" Mohammed.
As soon as he was a rich widower, a man with an army, he started killing. The "Medina" Mohammed. I wonder if all those wives made him really cranky.
And it is also a tenent of Islam that the later scriptures are more valid than the earlier ones.
Why? Does it do a better job than the Bible at demonstrating the existence of a deity? If it does, what I am supposed to make of that? What happens if I read the Koran, wind up believing in Allah and not in the Christian God? I never found the Bible convincing, even as a small child.