Some thoughts on the Winchester brothers.
The alignment of Sam with Hell and Dean with Heaven happens well before we learn that they are the true vessels.
Sam refers to seeing himself as unclean or tainted due to Azazel's (yellow eyes). As though by ingesting the blood of a demon he fell from personhood so something othered and twisted.
Dean is Sam's savior, he pulled Sam from the flames as a child and raised him in their father's absence. Dean sees himself (and Sam) as righteous and good as they save people and fight evil.
As Sam uses his abilities both he and Dean fear that he is straying from this righteous path, even when the visions are beyond his control. When Sam discovers that Azazel wants him to lead the armies of Hell we see this cemented in his character, from birth he was watched over by Hell's forces to become a weapon.
Dean acts as a counter balance even before the boys discover the existence of angels where Sam seems to have a knack for falling in love or in lust with monsters, Dean's interests lie in humanity (until the angels enter the chat (Anna/Cass)
When Sam starts drinking demon blood he unwittingly offers himself to Hell in a way, allowing the lifeblood of the damned to enter him of his own free will.
When Sam made the decision to leave John and Dean not once, not twice but three times he was mirroring Lucifer leaving the Kingdom of his father, feeling undervalued and slighted by the favor of his father being given to another.
Alternatively when Dean stayed he was the faithful soldier modeled after the heavenly host. Unwaveringly faithful to the father that had forsaken and banished his brother.