I really believe this is likely true. She had to have planned to kill him to cover up her abuse -- like a rapist who kills his victim to keep her from identifying him. And, she most likely dumped his body somewhere -- is there any non-frozen water nearby (hot springs or water-treatment plant)? Because, she would have a difficult time carrying him very far, or digging in frozen ground. She wouldn't have gone too far away from her vehicle, unless she made him walk to the spot where she killed him. Even then, not too far from a road.
A normal person (not really because normal people don't injure children), but a half-way normal abuser, would have seen what she'd done to the child, relent, and take him to a hospital -- perhaps lie about what happened. But, TS didn't -- instead, she doubled-down and killed the child.
I am haunted by a mental image of that helpless boy riding in the truck with his murderer, and going silently and innocently to his death.