From what I've heard this kid thought that he was torching a car for insurance reasons and he had no idea that there was a body in the trunk. This seems logical since he didn't even bother to hide it very well. Usually when you want to collect insurance on your car you would eventually want it to be found. This kid set the car on fire not far from the road knowing that it would be found but also knowing that it would be too late to salvage it, therefore insuring that Kelly could collect the insurance money.