[1.] You say the the reader isn't in a privileged position? That's an interesting statement.
[2.] As a child I was taught a maxim, a man is innocent until proven guilty. Now as a grown adult I know that is a fantasy. However it is the correct answer to your questions about murder and murderers. When you get to pick and choose, people get hung in trees and burned at the stake. [3.] I would love to hear a theory of the crime not using magic. Assuming that the police weren't too busy dealing with a Dark God wreaking havoc around the University. Why Harry, Butters, Cassius and a 200 pound killing machine broke and entered the Field Museum. How Cassius tortured Harry trying to steal a coin containing a fallen angel. Why Cassius doesn't exist record wise. What that funny book is all about. How Murphy is going to explain those pictures when the book ends up in the evidence locker and not in Marva's hand. And so on and so on.
PS This has nothing to do with the DF.
In 1988 I got a lesson in my hometown about the difference between knowing and proving. Look up a gentleman named Mel Ignatow.
1. I wouldn't say not privileged because we know Harry's thoughts, have no need to investigate, and get honest statements from witnesses. But we definitely don't have a "God mode" perspective.
2. Innocent until proven guilty
is a legal fiction. (One I believe strongly in). It has everything to do with the legal consequences of facts and nothing to do with the existence of the facts. Mr. Ignatow was just as guilty before the evidence was found as he was after. The only difference is that he was presumed innocent, tried with insufficient evidence, acquitted, and jeopardy had therefore attached. The our justice system has more to do with preserving rights than determining the truth. We keep true information from jurors all the time. Not because it's unfairly prejudicial, but because it wasn't properly obtained. My point is that a fact remains true regardless of the outcome of a trial. Harry's actions and intents are facts. We have a word for that constellation of facts. That word is murder.
3. What Kurstin said. But to go point by point, they broke in to steal an artifact the Nazis had looted in WWII, Cassius tortured Harry because Harry crippled him, specifically what Kurstin said about this point, valuable thing (who cares it's clearly "not real"), what photos and why do they need any explanation. A prosecutor doesn't really need to explain any of this. But all of that is besides the point because I think our main point of contention is that I think murder is a fact, and you think it is a legal conclusion or legal fiction, so it doesn't matter to me that a prosecution isn't going to occur or that it wouldn't be successful.
Plus, in many jurisdictions it's been abolished as a defense so if that is the case in Illinois Harry's lawyer couldn't even try to use it.
Self defense would be easier to prove.
But would the police seek out Harry to check for a blood-match? Why???
I just don't see how/why the cops would connect the evidence to Harry.
And even if they DID connect the evidence to Harry -- that evidence points to HARRY being attacked, and Harry's dog defending him... "We the jury find that Mouse is a Good Boy. Oh, and Dresdem is a snarky asshole, but probably Not Guilty." That whole thing about "beyond a reasonable doubt" is gonna be a tough row for any prosecutor to hoe.
Harry's blood has shown up at multiple crime scenes, including the scene of his attempted suicide by murder. If police took samples, submitted them to a state lab for testing and cataloging, and kept proper chain of custody, they would likely question Dresden about Cassius' murder. But as long as no one talks, getting a conviction would be near impossible. If Dresden, Butters, Michael, or maybe Murphy talked (I still don't recall if he confessed to her), a prosecutor would have a case that would likely end up in front of a jury. I find prosecutors to be overzealous.
This qualifies as the weirdest conversation I've ever been involved in.
Edit
During the course of this book Harry commits enough crimes to do time counted in tens of years. He shoots Luccio's body in the back of the head without any certain knowledge that Corpstaker was really in there. He breaks in and steals a valuable artifact from the museum. He breaks in to an electronics store. He destroys some poor persons car in the process of dropping it on Cowl. And he kills Cassius. I think that covers most of the major crimes and misdemeanors.
It could be argued that all of these (except Cassius) were either done out of self defense, defense of others, or necessity (private and/or public).
https://definitions.uslegal.com/n/necessity/ The gps would have to have been returned, though. I'd be happy to detail why if anyone cares.