Why would the honourable and good choice lead to a more evil Harry?
At the end of Grave Peril Harry has to choose between Susan resulting in war and peace on the vampires terms as most of the council would have preferred. I think the mirror Harry sacrificed Susan which led to a much darker Harry.
Because it has a whole mess of consequences, some of which are good short-term, but most of which are terrible in the long-term. My idea is as follows:
1. Michael went with Harry to get the Sword back. Harry wouldn't have been able to tear him away from Charity without it.
2. If Michael isn't there, Harry can claim Susan as a guest. They're able to vamoose from the party as a result, before the blood starts to spill.
3. Thomas and Justine are left to the vampires. Harry never meets his brother. Justine is eaten or turned. Thomas pretty much gives up or dies himself.
4. Lea is unable to offer a trade for the athame. As a Fae, she can either A) not accept it, or B) offer something else. If she offers something else, then swell, things move on as they did in Prime Harryverse. If she can't accept it, then the athame goes to someone else—maybe even Ferrovax. If Lea isn't infected, neither is Maeve, at least not through that transmission vector. This has obvious long-term effects; Mab isn't given Harry's debt, Maeve doesn't kill Lily—or Aurora isn't killed in the first place. Lloyd Slate may not even be made Winter Knight. Harry certainly isn't.
5. Major badness: the Red Court doesn't declare war too early, and take another five years to get their sucker punch ready. Instead of fighting a disunified, bickering Red Court wherein all the nobles are backstabbing one another, the White Council has to fight back against Pearl Harbors occurring ten times at once. The Reds run roughshod all over the White Council, taking pretty much everyone by surprise. Don't know what happens with Susan, but Harry is too busy putting out fires all over Chicago, constantly, and he's fighting a battle he can't win.
Dark Harry is created over YEARS of fighting a war that was lost the moment he started fighting.
6. Meanwhile, if Lea is and Summer Knight plays out the same way, the White Council isn't at war with the Red Court, so
they're not able to shanghai Harry into securing the Ways. Harry ignores the problem like he plans to do at the beginning. Maybe he just delays his involvement by a day. Harry's too late to stop Aurora. Or Mab goes to someone else to fix the problem, and they don't manage to get it done in time.
I can go on, but I'm pressed for time. Basically, my idea is that if Michael isn't at that party and Harry's able to leave, there's a huge ripple effect across the series timeline, and severely bad things happen as a result. Think about how Harry's involvement in the events of each book are absolutely necessary to avoid cataclysmic results, then tweak the details of what he does just enough, and things turn out totally differently, enough to the point that Harry grows a goatee.
