Well, first it is a what-do-we-do episode. Second, as a viewer, I really ddn't know what would come next, so that was nice. Would he relapse into a villain (albeit a somewhat sympathetic one)?
It might be said to be "about" the susceptibility to return to old ways/habits; and the power that is found in the purpose of service to others. Seeing Apollo grow even at his advanced age, and seeng Scotty grow out of his hatred were both very nice things. Very Star Trekky imho.
I think you have hit the nail on the head.... very well said.