Yes, making it a "favor" is SO COOL! And the last line evokes the image of "La Belle Dame Sans Merci."
I've always loved the idea of Snape as knight-errant, or rather trying to be a loyal man-at-arms even though all of those he's served - Lily, Voldemort, Dumbledore and now Harry - are fickle and fail to recognize and honor his service.
(Or it's just that we love a Snape who, like us, has read far too much Romantic poetry, Morte d'Arthur, Shakespeare, etc. etc. We do unrequited so well!)