Both Margot and Elliot have begun to change and have empathy for others. They used to be pretty self centered.
I think that basic decency was always there in potential. I mean, heck, Elliot was willing to take awkward, nerdy Quentin under his wing and be nice to him, which from a college student's standpoint is a pretty benevolent act. But being royalty has forced them to take on more responsibility and make important choices, and on the whole, they've made the right ones more often than not.
I am beginning to wonder why the fairies are really there. They are creepy but inconsistent.
That's pretty much the archetypal role of the fairy folk in myth and lore -- they're capricious and arbitrary, sometimes kind and sometimes cruel for unknowable reasons. (I saw a Facebook post the other day suggesting that animals see humans the way humans in folklore see the Fae -- we're vast and unknowable, we have incomprehensible powers, and sometimes we kill them for sport or as food, but at other times we lavish them with kindness and gifts, for reasons they can't understand.)