I saw something like this story on a forensics show called New Detectives on TLC. It showed a story about a fella who said his wife/girlfriend fell off of a horse and hit the back of her head.
He actually had injected her with something that killed her and then proceeded to smash the back of her head against a large rock to mimic the "fatal" injury.
I vaguely remember the motive being something along the lines of he didn't want to get married??
But Becky's tale is FAR creepier! You're lucky he wasn't crazy enough to take you and Mack too, chica!
Now - that could be what I saw. I remember the scene with her unconscious and him smashing her head on the rock.