Yeah, Windkin was super sweet in the "little palace" chapter - maybe it really was meeting Tyldak that sent him down the path of bitchiness?
Maybe he even sees a connection to his father in Door, and that's why he lets himself become attached (or whatever the right word is) to him? Also, even though they chain him up, don't the humans worship him like a god? That could also have affected his ego/self-image, right?
And yeah, that story with just him flying to Sorrow's End was amazing, so beautifully told - it's my favourite out of all the New Blood stories.
On a slightly evil note, I'd love for Tyldak and Windkin to meet, just so Tyldak can go, "Removable wings? He can fly just as high and as far as me, and he's still got usable arms? Why didn't I think of that?!"