@Tamerlane
Or his parents are killing each other while he's running away from abuse, or he's killed his parents and he's running away because he did a bad thing.
In either case, he's dying of weather exposure in cold rain, malnutrition, and injuries. The youkai encounters him in his delirious, feverish, dying moments. The only way she can make him her son is to make him "spirited away": let him die, or kill him.
Maybe I automatically reach for the most depressing interpretation.