But in that world, not the one you belong in.. Also you are assuming that Harry would make a different choice in the mirror world, Harry remains who he is, in both worlds. The effects on others might be different, but at the core, Harry is who he is. In Mirror Mirror, while the Kirk in the other world was evil, Kirk remained who he was in both worlds.
If people are WHO they are in strict sense, there would be no free will whatsoever - just nature playing itself just like Mab or Uriel does.
Free Will, Choice means - the character is able to take both choices, that he has in himself ability to choose.
And Harry definitely have some dark, dark potential - people would not faint at his Soulgaze otherwise.
The point for Butcher is to find possible choice earlier which chosen otherwise would gave us first step for Harry into deeper Dresden. Not a choice to turn him into mustacho twirling villain from the get go.
If Mirror Harry made a important free willed choice different from our Harry then he will be a different Harry. That kind of choices are the choices that shape your soul. That is why the butters tried to stop Harry when he tried to kill Rudolph, that choice would have resulted in a different Harry.
TBH one could agree Harry already made a choice to kill Rudolph. Butters changed consequences not choice itself.
But also in terms of souls - WOJ says otherwise - choices rarely changes soul - because soul already have potential of multiple choices in it's essence. And if you look into a soul - you see them - not just those active and realised - like with Molly's Soul - multiple fates, multiple possibilities, roads not yet taken, shadows of things...
It would change though - Sight. Sight is more about what's now around person. Soul is about immovable essence.
Or at least that's how I understand soul theory from WOJ.
So, again I hope it is that his choice did not made him evil but more vulnerable to external corruption and that is what make him evil (probably a Denarian??)
I think Darth Dresden will likely be necromancer.
Also we have seen Harry's condition after Grave Peril and that was when he saved Susan from an even worse fate, she could escape and handle her condition. If he had made the choice the white council had liked him to make and sacrificed Susan for "Peace" his condition would have been much worse.
And Harry is capable of making very bad decisions in such a condition.
Problem with this for me - is considering Harry state in "Grave Peril" I cannot see him at all submitting to peace offer.
It's... this thought does not even cross his mind really. I'd like to see something more subtle, smaller that leads to some catastrophe - possibly Michael's dead and Amoracchius destroyed or smth. I can more likely see Dresden fucking up more severly while trying to save Susan than leaving her for Reds.