He let her go. Near the end, when the police opened the door of the truck that had crashed into the house, we can see Charlotte in the back, tied up and gagged. Later we see her in the ambulance with Venna.
Rusty Nail is alive. The body the police found in the truck was not his. Earlier in the movie he hit the Ice truck driven by the guy who returned Lewis's Mastercard. At the end of the movie, the police say the body is that of "some guy who drove for an Ice company in Wyoming." Rusty Nail kept the body of the guy he hit, then switched bodies to make his getaway.