Jeremiah's older brother love for Swan and Caleb
It became very clear to me as the movie went on that Jeremiah Johnson developed deep love for Swan and Caleb, essentially as surrogate younger sister and much younger brother.
I really respected the platonic love between Jeremiah and Swan. They became close friends and then loved each other mostly like brother and sister. That was the best they could do with each other. Both were strangers unwillingly thrust together in an unwanted marriage because of Indian social customs and honor. But Jeremiah respects Swan and is the perfect gentleman, never forcing himself on her. In time, Swan acknowledges Jeremiah's respect and in turn develops her own respect for him. There is not a single spark of romantic love between them and it never develops but clearly you see some other form of affection and deep love, and it is most reminiscent of brother-sister love. As for Caleb, Jeremiah becomes father, uncle, big brother, legal guardian, and mentor to traumatized Caleb.
When you love family like that, it is understandable why a white-hot rage smolders within Jeremiah when he returns home to see their slain bodies in the cabin. At that moment, the quiet, shy, introverted, hard-working, honest Jeremiah Johnson dies and in his place arises a white-man demon avenging warrior, a man, who because he no longer fears death nor harbors any concern for his own life now becomes invincible for it. He will no longer be known as the white man trapper trader to the local Indian tribes but a yellow-haired, ice-blue eyed killing demon from out of the East, who breathes and bleeds but never dies and fights as raging demon, enough to become a legend in his own time.