Here's my theory of what was their motive and what happened:
They didn't go to Whitehorse to find work. They were doing something illegal. I don't know what, maybe drugs or guns... something we may never know but definitely illegal. They had the evidence of their crime in their car, and were scared that the police might find the evidence and they get into trouble. They wanted to somehow get rid of their car, but they needed to steal another car first. They saw Lucas and Chynna's van parked by the highway. At first they didn't see the owner's around, and proceeded to steal the car. Suddenly Lucas and Chynna show up. Being the scared and inexperienced teens that they were, they shot the couple to death. Then they broke the windows of the van, opened its door and tried to start the engine... ****. The van doesn't work. Now they have murdered two people (without originally having the intention), a broken van, and their own car with whatever evidence in it, which they are trying so hard to destroy.
Now they are even more scared. They think there's no evidence to show that they were the murderers. They immediately jump into their car and try to get away as fast as they can. Later, they find another car by the highway, a RAV4, and an old gentleman chilling around. Having already murdered two people and nothing to lose, they kill the gentleman and steal his car. Again thinking that they have left no evidence and can get away with it if they drive really fast and get as far away as possible. They torch their own car, successfully destroy the evidence of what we will probably never know, and drive towards east.
They drive and drive and drive. Nonstop. They still have some hope that they may get away with all they have done. They're probably trying to make up a story to explain their torched car and the route they are taking. They have stole their victims IDs, just to buy themselves some time before the police can identify them and gather more information. At some point, when they are in northern Manitoba, they realize that they are stuck. The police and public know about them, know their faces and actively searching for them.
At this point, they're just too scared and don't really know what to do. They probably tried a few different scenarios: kill another innocent and get their car, hitchhike, etc. Finally, they decide to again torch the RAV4, destroy the evidance of something we may never know, and hide in the woods and wait until the police give up searching for them, or maybe meanwhile try to reach somewhere they can survive without being identified. Whatever their plan was, it went wrong and they ended up dying in the woods.
We may never know the details. But I believe they didn't just randomly decided to go and kill people. They ended up in a situation, got desperate and had to murder people, and after that they just had to go down and down. Kind of like Walter White in Breaking Bad. I don't think they killed people for mental issues or just for the sake of killing. They didn't harm the man who helped them get their car out of the mud although they had the perfect opportunity. I don't think they were evil serial killers. Just two kids (yes, kids) going after some dangerous adventure, and things went horribly wrong. Five lives wasted. What a shame...