The violence was not overdone by any means. Though the story of Hugh glass was not portrayed factually accurately, I find that the violence of the time was. However, I don’t believe the rape story. I’ve read everything I can so far on the fur trapping Era, but have never heard of an incident written down where a native girl was taken as a sex slave.
In the book, she wasn’t a slave - more like someone who opted for a bad life (other options being worse).