It's times like this when I really appreciate good critiques. You and Smartry have given me the first big, in-depth critiques I've had in a very long time. My parents aren't too fond of fantasy, nor are they writers, and the only writing group in my area is dead.

The setting and the interactions do come across as rather original, too, that said, so please don't take this entire review as one big downer.
Don't worry about making me feel down. I get so few fiction critiques that I don't have the luxury to feel anything but absolute joy when I actually get one, no matter how harsh it is. So no, none of the three critiques was a downer. I felt nothing but absolute joy. Seriously.

Erm, isn't this kind of rediculously, incredibly lienient? If this was a human king, and the human mistrust of angels and feykind is a officially state-sponsored thing, that should really, really be brought up here. Because even then, I figure they would not be inclined to listen to a confirmed murderer's pleas, esspecially not if they were also a fey themselves. A young age doesn't even begin to cover why she is apparently walking around, free, no strings attached. I certainly don't think she would ever shake that stigma off around the local populace. >_>

The only ways I can really think of rectifying this would be if either; A) The King was completely corrupt and morally absent himself, and was intent on weaponising her in future for his own use. B) The angel killings were part of the revolution itself, and she only murdered a few amoungst the chaos, alongside other racist individuals. I kinda prefer this version since it is much less mary sue-ish...
Yeah, Siren should probably be dead. She killed an angel, and killed nine more in pseudo-self-defense when they attacked her in retaliation.

Siren's back story is kinda vague, even to me--though a bunch of racist killings in addition to the king/governor using the kid as a tool of war is a good idea. Even so, she shouldn't be running free unless I have some super-good explanation cooked up, and I have a bit of one:
Siren's sentence is actually the topic of a game I'm developing for a final project in Intro to Game Programming. The main mechanic of the game is interpersonal communication: Siren does her killings and the court postpones her execution for a month while they gather evidence. During that time, Siren has to make sure more than half of Sigmora wants her to stay alive. She could do this by making the city dependent on her (pseudo-canonically, she led the rippers to Sigmora in the first place), and/or she could either manipulate or persuade the people--and therefore, the jury--to think highly of her.

The big problem, as you already brought up, is why in the world she's allowed free reign that whole time. I'm thinking she'll be suspected, but not actually caught committing her crimes, and in a month when the court re-convenes, she knows they'll have the evidence to sentence her. If I go that way, that brings up another gameplay option: tampering with evidence.

Tendons are made of collagen, also, just in case you didn't know...
Hm. The narration is Illyana's, and I assumed Illyana would know almost nothing about anatomy. Is there a way I could pull off the same thing and be less jarring about it?

Anyway, thanks for the critiques. I really appreciate them. I'll rewrite everything accordingly and give you a much better story. You can give chapter 4 a thrashing if you want (it needs it), and I'll definitely implement any necessary changes, but if you'd like to wait for me to rewrite it based on Smartry's critiques and have a story that isn't painful to read as a result, you can do that too.

(I should also stick to non-fiction on my blog if my story's canon is going to change this much.)