The Hitcher

Jim Halsey and Grace Andrews are driving across New Mexico to meet her friends for spring break. One night in the pouring rain, they almost hit a hitchhiker who is standing in the middle of the road near his broken down car. Jim swerves and the car spins out of control to a stop. As the man approaches, Grace insists that someone else will stop to help him and they speed off.
At a gas station, Jim sees the hitchhiker climb out of a truck. Inside, he introduces himself as John Ryder and asks for a ride. Reluctantly, Jim agrees. While on the road, the hitchhiker becomes violent and attacks them, holding a knife to Grace's eye. The hitchhiker tells Jim the only way to save them both is to say, "I want to die." Jim hits the brakes, causing the hitchhiker to hit his head on the windshield and kicks the hitchhiker repeatedly until Grace opens the passenger door and he is pushed out.
The following day while driving, a family in a station wagon pass them, with the hitchiker in the back. Jim and Grace try to warn the family, but crash their car. The couple is forced to continue on foot and they eventually find the family's car on the side of the road; both the children and mother are already dead, with the father badly wounded and slowly dying. Taking the station wagon, the couple head towards town hoping to take the man to a hospital. After the hitchhiker attempts to run them off the road, Jim and Grace stop at a cafe, where the man dies.
Suspected of committing the murders, Jim and Grace are arrested and brought to the police station. The hitchhiker arrives shortly after and kills everyone at the station except for Jim and Grace, who flee. Despite suspecting that there might be a third suspect, Lieutenant Esteridge continues to pursue the couple. The hitchhiker shows up and helps Jim and Grace escape the police, single-handedly take out all of the police cruisers and helicopter.
Grace and Jim arrive at a motel and Jim steps out of the room to make a phone call. He is gone for several hours and Grace falls asleep watching TV, but is woken by the hitchhiker, who tries to rape her. Grace manages to fight him off and hides in the bathroom.
The hitchhiker disappears and Grace searches for Jim. The motel manager sees her with a gun, and calls the police. Grace finds Jim who is chained between a truck and a trailer. Grace runs to the truck and points the gun at the hitchhiker, demanding he stop. The police arrive soon after (still thinking Grace and Jim are the killers), and see Grace holding the hitchhiker at gunpoint, but realize Jim is being tortured. The police tell her to drop the gun, and the hitchhiker tells Grace not to listen to them before driving forward and splitting Jim in half, killing him. The hitchhiker and Grace are apprehended by the police.
The next morning, Esteridge tells Grace, whose innocence has been proven, that the real John Ryder is missing and they do not know the true identity of the hitchhiker. He also informs her that he will be taking her to her parents and the hitchhiker will be transported across the state in a separate van. During the journey, the hitchhiker breaks free from his restraints, kills the guard who is watching him, and causes the van to crash after the driver loses control, also causing Esteridge and Grace to crash.
Esteridge is trapped in the car and Grace takes Esteridge's gun, and slowly makes her way to the van. Grace opens the van and the hitchhiker attacks her and locks her in the back. The hitchhiker shoots a pool of gasoline near the van, igniting it. Grace manages to get a shotgun from the front seat of the van and escapes as the van explodes. The hitchhiker shoots and kills Esteridge and walks away. However, Grace shoots the hitchhiker, killing him and ending his rampage.

Question: Who is she trying to save?
Answer: Jim