Faeries (be they pixies or sidhe) do not have freewill. That is why the Queens have Knights. The Queens can't harm anyone not associated with one of the Faerie courts, they may want to kill someone (they can even tell people they want it done), but they can't. That is what the Knights are for. They can have the Knights (which are mortals with freewill) go off whoever.
Their lack of freewill is evident by their binding to oaths. Faeries
can't break their oaths once made. They just can't. That means they can't make an oath they can't fulfill.
Most of the Sidhe seem to have "freewill" but really they have very broad high concept aspects (and probably a few other aspects, I imagine "common" faeries just have a high concept, a trouble, and maybe 1 other aspect). Lea definitely has some aspect related to acquiring power (
this aspect forced her to acquire the black athema which did terri-bad things to her mind, hence the white streak in her hair
) and looking at her "stats" sure enough I found "Power is my drug of choice."
But power is very generic, and as Faeries see it, it takes many forms (personal power, favors, possessions, position, influence, money, territory, even allies). A faerie may seem to act contrary to its nature because it is fulfilling its end of a bargain.
(Small Favors spoiler)
Mab makes Harry forget about using fire magic and takes his blasting rod so that it is harder for Summer to find him in Small Favors. This technically makes Harry a weaker emissary to fulfill a task she set before him (which seems opposed to her nature), however it makes it more likely that Harry won't die (and therefore fail a task for Mab) which justifies it.
Being a Changling isn't that bad. It
is the closest you'll get to "Faeries with Freewill." They can even be entangled in Court Politics (Changlings are associated with Faerie if not one of the courts just by virtue of a parent). And if their Faerie parent is female, there is a good chance they were raised partially by the parent (to me Changlings represent "poor man" knights, i.e. a Queen can beat on them without using her Knight and they have Freewill so they can do some Freewill tasks).
Why else would
Maeve be so interested in Dresden knocking up her Nixie handmaiden? She wants a wizard--although I suspect she specifically wants a child with Dresden's "anti-outsider mojo" potential--that she can influence. Wizards aren't just powerful--Maeve has power--they have power AND freewill--and a lot of creatures seem to know about Dresden's "auspicious birth/omg he can pwn an Outsider backed dude." Heck! She even offered to service Dresden herself!