The Hitcher

Jim Halsey, a young man delivering a car from Chicago to San Diego, spots a man hitchhiking in the West Texas desert and gives him a ride. The hitcher, John Ryder, is brooding and evasive; when Jim passes a stranded car, however, Ryder forces his leg down on the accelerator. Ryder calmly states he murdered the driver and intends to do the same to Jim. Terrified, Jim asks what Ryder wants. He replies, "I want you to stop me." Ryder strokes Jim's face with a knife while telling him to say, "I want to die." When Jim realizes that Ryder never put on his seat belt and the car's passenger door is ajar, he shoves him out of the door.
Relieved, Jim continues on his journey. When he sees Ryder in the back of a family car, Jim tries to warn them but becomes involved in an accident. He later comes across the family's blood-soaked car and vomits. At a nearby abandoned gas station, Ryder corners Jim but simply tosses him the keys he took from Jim's car. After Ryder leaves with a trucker, Jim encounters him again at another gas station, where the trucks nearly runs him down as it crashes into the pumps. As Jim flees, Ryder ignites the spilled gas and causes the station to explode.
At a roadside diner, Jim meets Nash, a waitress, and calls the police. As Jim relaxes, he finds a severed finger in his food and realizes Ryder is present. The police arrive and, after finding Ryder's knife in Jim's pocket, arrest him. Though the police doubt his guilt, they lock him up overnight as protocol. When Jim wakes, he finds the cell door is unlocked, and all the officers are dead. Jim panics, takes a pistol, and flees. At a gas station, he sees two officers, takes them hostage, and speaks to Captain Esteridge on the radio. As Esteridge convinces Jim to surrender, Ryder pulls up alongside them and kills the two officers.
The patrol car crashes, and Ryder disappears again. After briefly considering suicide, Jim reaches a cafe, where Ryder suddenly confronts him. After pointing out Jim's pistol is unloaded, Ryder leaves him several bullets and departs. Jim boards a bus, where he meets Nash and attempts to explain his situation. After a police car pulls over the bus, Jim surrenders, and the furious officers accuse him of killing their colleagues. As the officers goad Jim into a fight, Nash appears with Jim's pistol, disarms the officers, and flees with Jim in their patrol car. As the police chase after them, their car crashes, and Ryder shoots down a helicopter, causing the police cars to also crash.
Jim and Nash abandon the patrol car and hike to a motel. While Jim is in the shower, Ryder abducts Nash. While Jim searches for her, Captain Esteridge takes him to two trucks with Nash tied between them. Ryder is at the wheel of one truck and threatens to tear Nash apart. Esteridge tells Jim that his men cannot shoot Ryder as his foot will slip off the clutch, which would cause the truck to roll and kill Nash. Jim enters the cab with Ryder, who gives him a pistol and tells him to shoot, but Jim is unable. Ryder, disappointed, releases the clutch, killing Nash.
Ryder is taken into custody. Esteridge gives Jim a ride, but, having no confidence in the police, Jim suddenly takes the officer's pistol and vehicle to chase down Ryder's prison bus. As Jim catches up, Ryder arms himself with a shotgun, kills the deputies, and leaps through Jim's windshield as the bus crashes. Jim slams on his brakes, sending Ryder through the windshield and onto the road in front of Jim's stalled vehicle. As Ryder shoots at him, Jim frantically restarts the car. Ryder challenges Jim to run him over, which he does. As Jim leaves his car to observe Ryder's lifeless corpse, Ryder suddenly jumps up, and Jim shoots him. As the sun sets, Jim realizes his ordeal is finally over.

Question: Who is Jim chasing?
Answer: Ryder.