Looks like a torched car gone wrong, I am sure that burning a car 75k from civilization was planned thinking no one would find it. They had to destroy it, it was evidence against them, it could not have been "left" They were destroying fingerprints and DNA. Same thing they did to their own vehicle, they had a lot of time to scheme on that long drive across Canada, Kids like to burn things.
They may have been 75Km from the "civilization" that is Gillam, MB but they were only a couple of Km from some of the First Nation homes around Sundance. I also think it was a relatively clear day. If they had known the weather, they should have waited for the foggy or rainy days that were just around the corner. A serious miscalculation if they thought they were going to be able to conceal it