A carjacker is the person who does that crime — the one who stops a car and, with a weapon or a threat, makes the driver give up the car or drive somewhere. The crime is carjacking; the person is the carjacker. You will meet the word in news stories. It is not a job title, and nobody says it with pride: it names someone who has used fear against a driver. Though actually, the ending looks like the same -er you see in teacher or baker, but here it does not make the act sound ordinary or kind. It only means “the one who does this.”