Complex sentence: He uses the truck to determine the suitability of his victims, bumping into them from behind to frighten them, this is so that he can smell their fear, and pick out what part of the human he wants.
Split into simpler sentences: He uses the truck to determine the suitability of his victims, bumping into them from behind to frighten them. He does this so he can smell their fear, and pick out what part of the human he wants.