I really liked it, it was quite saddening to say the least. It's plot is refreshingly simple, and if you can't take that as a complement, I wouldn't know what to say, but what am I really to say? It's just the prologue so far.
On the topic of writing style, it's just the way I like it for this kind of story, quick and to the point, and also knowing when to slow things down or speed it up.
But even then, I think a LITTLE bit more description on the environment and Thorn himself would be good, but I'll leave you, the author, to decide when to fully reveal Thorn's nature.
Also, I think Grey Feather's dialogue is a little bit... 'empty', I'm fully aware that is Grey's nature, but even so, I feel he needs to sound a bit more superior, or have a more defined personality. Another way you could improve Grey Feather is by detailing a bit on his actions, or expressions
-- "You’ve been causing so much trouble for me; with your raids on my drug and weapon deals. I thought after all that you had taken from me I should take from you. But you have no desire for worldly possessions so I'll take something you do care about," and with one swift motion Gray Feather brought a knife down into Summer Breezes throat. --
You could improve this by maybe throwing exclamation marks or emphasize some words, or even make him snort, spit or any other physical manifestation to show his superiority and disrespect over Thorn or current emotion, or simply how annoyed or frustrated he is.
Also, grammar errors. I am not fully inclined or experienced to correct grammar errors, I am not even sure if half of what's below is an error, so don't take what's below to heart. I am in no position to force these changes on you.